I cannot give you an yes or no answer to that . After reading the opengameart topic and some others my understanding is that it may not be as cc-by-sa clearly states that all derivate work should have the same license. I am 100% certain that you cannot make such linking in single files.
The rest is not clearly defined because game is a whole you cannot just turn of some graphic files and, run it without. Take in consideration that some people question even using cc-by-sa art with gpl engine (vcmi is gpl). However imho code is separate and one could always defend one self that vcmi can work with other graphics, also there are a lot of foss games that use gpl for code and cc-by-sa for asets. However I have not seen any big projects that mixes gpl art files and cc-by-sa files. All go for one license for their files.
If you consider simple screenshots from the game. Those are derivate work and are in one file. So a screenshot of gpl adventure map and cc-by-sa interface is not legal. If you think about it then the game art as a whole really defends itself.
So to sum up I’m not an legal expert. I decided to use one license cc-by-sa to be safe, and because there were more art assets with that license that I needed. Secondly all the big project stick with one license. Sadly this means some really nice gpl art cannot be used by me
I would suggest to do the same with your project, because it might really hurt you when someone requests his art out of your project. How will you defend yourself then ? Give a link to other projects that also mix gpl art with cc-by-sa ? Might be hard to do.
PS. This cc-by-sa relicensing to gpl is interesting. since 2.0 cc-by-sa is also relicensable to any former license. This probably means that all 2.0 and up cc-by-sa can be re licensed to 4.0 and then relicensed to GPL. However I’m not an expert so it a big if…
Also there is a lot of art that is gpl 2.0 only without “or later” so here is another incompatibility.
Ivan I know I was the one that cried two years ago for having this support but right now I’m not sure. Town screen windows will clearly benefit from high color background (buildings maybe to) The same can be said about battlefield background but probably one can use high color bmp fot it even now. I’m really unsure now about adventure map as I have not tried to replace any objects.
What I have tried however is creature animation and I think all of my units will be fine with 256 color and will not loose much . I’ve used H3DefTool and it was also possible to make a def out of 24bit bmp frames … and vcmi was able to use such file. The only problem was the special colors. Both transparency and shadows didn’t work. Maybe fixing that would be quickier and easier and you can claim such support in VCMI without the need to re write massive amounts of code?
The other thing to conider is the tools. Many years ago I tried doing defs and the tools were horrible, but now ? H3DefTool is quite nice. It lets you set/change the position of unit for all frames. It generates shadow for all frames !. For buildings in town it generates the outlines…
Well I am for certain sold. Doing those manually is a lot work and with pngs those things would have to be done frame by frame …For adventure map this would be doable but for creatures anims? well imho no… So yet again maybe some fixes to code to make those 24 bmps in defs work fully would be better and easier to do ? and if anything pngs on adventure map would be more usefull then creature anims IMHO.
PS . Sorry for sp errors and typos. I had one functional hand to type all that.