It doesn't related to the jam in any way. And I allow on my jam any license and even rate without looking at license... Just when I see one of the free cultural license
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Kenney is very popular
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In general you can write something like this:
License:
- You can do X
- You can do Y
- You can't do Z
And it will be "considered" as license so you can use your any custom license. In general it consists of following parts
- Must user attribute the author?
- Can user modify the art to suit their needs?
- Can user redistribute the art?
- Can user use it commercially or only non-commercial use is allowed
But some creators use free cultural licenses usually from CC (Creative Commons) family