Yes, you can use art under a free culture license in a paid game. Just beware that:

1. Most if not all of those licenses mandate that people who get the game must be able to extract and reuse the artwork with relative ease.
2. GPL-licensed artwork *might* be viral, in other words the license might extend to your game. I'm not sure. That's not the case for CC-BY-SA.

And no, you won't have to pay the artists either. Though that would be nice if you make any money.

But seriously, if you need to worry about legalities, don't ask us. Ask a lawyer. Any advice you can get here has no legal value.

In simple words? If in doubt, avoid GPL assets. And to make sure, ask a lawyer.

If in doubt, contact the artist directly (and privately) and get them to sign a release agreement, and negotiate the license terms as needed. You shouldn't really need a lawyer for that as there are viable templates available online.

Using GPL licensed artwork requires you to release your game under GPL too, which is a problem if it's supposed to be commercial. I heard it was Microsoft who established the term 'viral' for this. It's pretty much the same, if you use CC BY-SA assets.

You'll be on the safe side using CC BY 3.0/4.0 and CC0 assets though.

In reality, not many of us can afford legitimate legal advice. Maybe I'm too cynical but most of them would just love to throw you into an infinite black hole of ambiguities and contradictions, while charging you by the hour...

Just reach out to the artist and like I said, appropriate a template for a release agreement. They are pretty standard. It will make sure you have non-exclusive commercial rights and put, at least in part, liability on all signators. Anyone who is genuine and confident about their work should have no problem signing it for you.

Ultimately, you're never really going to be sure unless you're working with artists in a physical studio and watching them throughout the creation process, usually given a set of licensed resources and software particular to that studio, so they can be 100% sure, and even then there's a lot of paperwork and contracts flying around.

Sometimes an artist may use resources (carelessly but innocently enough just Googled) and unwittingly violate copyright, even something as simple as a rivet or screw, as part of a bigger work. The only way to be sure is to have a release agreement with some mutual liability. Then, you actually have something to take to a lawyer to notorise, which _should_ take less than 1 hour...

It can get messy and depressing. However, by all means - never let any of that dissuade anyone from continuing to try!

"Using GPL licensed artwork requires you to release your game under GPL too,..."

Debatable (though I'd rather not). The Free Software Foundation has clarified that use of art does not trigger the GPL's linking requirement. In most cases, the artwork must also be GPL, but the game code need not be. This kind of "well technically..." response is ultimately why claudeb recommends avoiding GPL entirely and why LDAsh recommends consulting the author directly... and it's why I agree with both of them. :/

Hey MedicineStorm, do you happen to have a link for where the FSF issues this guidance about art not triggering the linking requirement? I've seen that fact mentioned before but couldn't find it on their website.

This discussion regarding a conversation with FSF is a start: http://wesnoth.org/forum/viewtopic.php?p=108802#108802
Also, this blurb from the FSF FAQ:

https://www.gnu.org/licenses/gpl-faq.html "...some programs, particularly video games, can have artwork/audio that is licensed separately from the underlying GPLed game...."

Admittedly, the scenarios outlined are GPL code using non-GPL art, not the other way around.

I am confident there was a clarification that Bart was basing our own FAQ on, but it may have been between Bart and FSF, not a public forum. Looking for other references...

Please note I say "GPL artwork requires GPL code is debatable", not "saying GPL artwork requires GPL code is incorrect".

At least in part, it depends on...

1. ... how the art collection is packaged in the game,
2. ... how the author wants their work to be distributed.

One of the main arguments is in #1, where artwork is packaged as game data which is separate from game code. Data in GPL'd applications do not need to be GPL as well, and conversely GPL'd data does not require the application that loads it to be GPL as well.

For example: Loading GPL graphics (the data) into Photoshop (non-GPL code) is not a violation. The derivative output (modified graphics) is still GPL, but Photoshop itself doesn't need to be GPL code just for having loaded GPL data. This isn't a perfect example since it is user-loaded GPL'd data and we're talking about developer loaded GLP'd data, but it illustrates what I'm referring to when I say Data vs Code.

There are some instances, depending on how your game code deals with assets, that the code should be GPL due to the way the GPL assets are incorporated into the game's code, but most game engines can load assets unincorporated or as separate collections of data. This is not intended as a loophole, but it does make a difference and I would even say it is how the GPL is intended to operate: GPL affects code, but not data, and art assets are usually data.

The argument for #2 is that you want to respect the artists wishes regardless of the technical interpretation of the license. It is debatable either way, which means it isn't 100% clear in all situations. Therefore:

• If you are a developer seeking to use GPL assets in your non-GPL project, you should ask the artist if they're cool with that. You can still get sued by a pissed-off artist even if your lawyer says you're right.
• If you're an artist that would like your GPL work to only be used with GPL code, you should say so on your asset submission page. Just setting the license to GPL is no guarantee the downloader will know that it should only be used with GPL code.

I'm not sure where this GPL confusion came from. GPL was designed to infiltrate the code base of the program itself, i.e. all stuff creating the final binary and the libraries it calls. It doesn't apply to external data, or external programs. External - meaning there is no function call. You can run GPL program and read its output from closed source program. And the opposite - GPL program can read proprietary data. There is one exception though. If the data is a script, and the script calls GPL code, then the script "links" with GPL, and therefore must be GPL. But art assets just can't "link" with source code from the perspective of GPL.

However, most, if not all believe that GPL will "leak" into the game code itself, since they've heart that this is what GPL does... This is why one should definitely avoid it if the whole project would not be released under GPL. Even with permission of the author, what about gamers that look into this and "feel" it's unfair. Making something is not just about the law, but also how people perceive your work. Especially the GPL guys are very "touchy"...

But IMO, GPL is a terrible license for the kind of people that will search for free assets. These are generally indies, one or few people that will have difficult time "protecting" a fully GPL game. Why "protecting"?... The biggest strength and biggest weakness of GPL is the ability of forking. Forking allows the project to live on, no matter what happens with the original author, but also means that anyone can make a spin off of your work, making it near impossible to have something like an IP. This is generally fine for most software (there you can patent), but when you are creating a game, you have particular vision. Imagine someone else taking the source, and doing all sort of stuff with your game/vision. Many would not like it. It doesn't mean that the author doesn't like to open source the game, or want to sell it. It's about GPL particularly, which was not designed to work for art / subjective topics.

I can give the following brutal example. Imagine you create some priest/nun assets and someone else use them to create spicy xxx content. You can't avoid similar things from happening with GPL, nor CC-BY-SA. Open source generally works best for bigger companies that can support their mainline code. For example Blender. There were a couple of Blender forks, but failed since the Blender Foundation has bigger community and more resources. Especially these days... But for indie devs, GPL means fragmentation of the community, and potentially - loss against another fork. Same with CC-BY-SA.

IMO, this is why Unity/UE4 became so popular with indies. Actually - these are easier as platform. We get basically free engine, doesn't need to release any code. There are many free assets, or cheap ones to buy. For $100 buck you can do simple games, and do not have to deal with all the drama. At least for games, this seems to work best.

What if you are not even an indie, but just a hobbyist? This is probably the toughest situation. Dwarf Fortress is one great examples - free, but closed source. If it was released open source, we would have at least 100 clones by now that very little gamers would be able to differentiate...