You can use Battle for Wesnoth art for free if you make your project open-source. Just an idea.
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It's a good idea. I considered it a few months ago, to gain access to a collection of Lovecraftian images for backgrounds/spell depictions. I might go down this path, depending on how things work out. It would mean that I can't use Unity Asset store code, though, which I am currently using for graphs, so there would be advantages and disadvantages.