Unfortunately there is no easy solution for this situation but you can do the following:
Get a permission for using the art (Already done by doudoulolita)
If there are any troubles with third parties get a formal permission like this:
I <author's name> grant <user's name> permission to use my art asset <art asset's name> in his/her commercial game <game's name>. This permission expires never and can only be withdrawn by myself <author's name>.
The art can be used under the rules of one of the licenses.
CC-BY-SA 3.0 OR GPL 3.0 OR GPL 2.0
The license and the SA clause are only applied on the art not on the source code. So you have to distribute the art under the same condidtions as here (free distribution, mention the author, etc.). You can license your source code like you wish (OSS or CS).
Any chance to get new art assets into the FLARE game?
Unfortunately there is no easy solution for this situation but you can do the following:
I <author's name> grant <user's name> permission to use my art asset <art asset's name> in his/her commercial game <game's name>. This permission expires never and can only be withdrawn by myself <author's name>.
The permission has to be signed of course.
I hope that helps. :-)
The art can be used under the rules of one of the licenses.
CC-BY-SA 3.0 OR GPL 3.0 OR GPL 2.0
The license and the SA clause are only applied on the art not on the source code. So you have to distribute the art under the same condidtions as here (free distribution, mention the author, etc.). You can license your source code like you wish (OSS or CS).
I hope that clarifies that for you. :-)
The art is licensed as CC-BY-SA 3.0 and can be used without opening your code.
Your game is really cool! Nice work. :-)