Yes you can.

You have to be aware of the licenses though. Some of the art require attribution and i believe some of the licenses are share-alike licenses

@snkioggs:

You maybe ok to use works on this site in your project, but it depends on the works are licensed.  You can see which license(s) the artist has released a work under by looking at the icons under 'License:' on the upper left of the page for viewing/downloading the work.   If a work is released under multiple licenses, you may choose to use it under the terms of the license which best suits your needs. 

As it happens, I was just working on drafting a new license doc for the site which summarizes each of the licenses that are supported here.  You can find the draft here:

http://opengameart.org/forumtopic/site-faqsubmission-guidelines-updatesc...

You'll need to scroll down a bit to get there, but the draft contains a brief overview of each license and what it means in terms of commercial use.  There are also links to the full text of each license.

Also a quick note, with regard to your app being free vs paid vs in-app purchases, it's generally doesn't matter what you're charging or how, or even if you're using the work commercially or not, what matters is /how/ you distribute the work (eg. do you credit the original author?  do you include a copy of the work's license?  can you apply DRM? etc. etc.)

Hope that helps!

Actually, do let me know if the license overviews in the draft are helpful or not.  The point of them is to try and help folks in exactly your situation (eg. new to licensing issues) to quickly get at least a basic understanding of the different licenses used here.

@snikoggs:  Thanks glad to know the doc was helpful!

> I already downloaded some images a while ago and forgot to note them to read about their licenses.

Yeah, I think this is a pretty common occurance.  We're working on making the licensing info more prominent and accessible for new users, as you can imagine it's a fine line between making sure people understand the different license options and smothering them in legalese.

I don't get "share alike resources " can somebody please explain? :(

Oops I meant license, not resource.  

I had a go at explaining this in lamens terms in my re-work of the site faq/docs:

http://opengameart.org/forumtopic/site-faqsubmission-guidelines-updatesc...

Here's the language about share-alike for CC-SA-3.0:

2) If you make derivative works, you must distribute them under the same license (CC-SA-3.0) 

   Derivative works inlcude modifications of the original work, but do not include entire projects or games which merely use the work in it's original form.

If you are using art, that means commercial use is ok, so long as you provide appropriate credit, don't distribute the work in a way that includes DRM, and release any derivative works as CC-SA 3.0.  

If you are submitting work, this licenses means people are free to use your work but must credit you as it's author, can not use it on platforms that impose some form of DRM, and must distribute any changes they make under the same license.

Does that help clarify things at all?   Curious if it helps because the goal of my effort on this is to make the site docs/faqs more clear for people not yet steeped in license babble.  :)

Very well explained capbros. Im glad you're here. I was actually a bit fuzzy on a few parts myself. Maybe we should try to make a sticky forum post in this section for newcomers on the site to see?

Making this a sticky forum would be a great idea. Before this post I didn't have a clue about all the different licenses. 

@Chris_M_The_Game_Dude:

Glad you found it helpful!   Nota buena though, be sure to read the discussion at the bottom of the feedback forum post:

http://opengameart.org/forumtopic/site-faqsubmission-guidelines-updatesc...

mdwh has correctly raised a concern about whether games qualify as 'Adaptations/Derivatives' or 'Collections' with regards to the 'share alike' clause CC-BY-SA licenses.  I have contacted the Creative Commons team to try and get some clarification on the issue, but their initial response was very much 'it depends', implying that CC-BY-SA assets should not be used in closed source projects unless you're prepared for the possibility that you may ultimately have to share your source.

Making this a sticky post would be a good start, the ultimate goal is to get the site docs updated.  But even then maybe a sticky post that directs people to the docs/FAQ would be good.

Yes. I think we should get detailed info for each of the sites licensed though. Im sure we can all say other than the 'CC' licences most of them are pretty vague. Ill try to educate myself a bit more on the licencing details to see if I can help out or soeed up the process a bit. Personally the licences from GNU confuse me the most.

Or the OGA-by 3.0, bart put up very little info on it and even if he did it doesnt seem to hold any legal standing, more suggestive. Even though all licences are just suggestive without pricy legal representation.