There’s an interesting discussion over in the Board Game Geek Board Game Designers Art & Graphics forum about whether it’s ethical to use someone else’s art in your prototypes.
It’s not legal, that’s fairly clear. Generally speaking you’re not allowed to use any part of someone else’s copyrighted works without explicit permission. But since copyright and IP law differs from country to country you’re operating in a gray area. For example in the US, where copyright law focuses almost solely on ownership and economic gain, you might get away with using someone else’s art as fair use. In Europe, in contract, you wouldn’t, no matter that you didn’t make any financial gain nor impacted on the artist’s financial gain. In Europe your copyright is protected no matter what and your right to be named the creator of the art is firm (although we don’t have quite as rabid copyright legislation nor as high punishments as in the States, but that’s changing).
But let’s talk about it in moral terms: is it right to take someone else’s creations in order to prettify our own?