I am not a judge, but the licensing looks good to me - it's the basic GPL header, which does the trick.  I'll try to play it later today when my daughter gives me the time.

I'll just ask my question here since this topic is appropriate. In our game we have .gor files which includes assets. .gor files help us to create proper structure for loading etc. Since all assets are in .gor files doing something like fallowing is okay?

blabla.gor file includes

- good_tree.png (Martin Goodartist)

- cool_house.png (Jeremy Coolartist ) 

Again, I am not a judge, but the way I see it is:  You want to make sure people know you relied on Martin's and Jeremy's work, but you also want to let people know that you modified their work so that nobody blames a crummy color swap, scale, or crop on them.  If good_tree.png and cool_house.png are not files in your game, probably link back to where you found those files (I linked to the entire LPC art asset .tar.gz).  

If there's an "official" way to do it, I'd love to hear it!  If there isn't an official way to do it, OpenGameArt could probably do with a guideline of some sort.

Anything without a clear license attached to it, is assumed to be free as in freedom. I believe it's that simple, and that's why you have to accept that before uploading your game.

It's simple if it's your work, but you have to be careful if you took something from another place... that's all.