Hello. For license, we need more than an agreement to use, but an agreement to release the image into a free license. With a free license, UFO:AI can use it, but also everybody (not only a game, not only an open and free project) while they respect the license (often author credited (by), and often derivative work into the same license (share-alike)). We can use image in CC-BY-SA, CC-SA, Public domain, or GPL (and much more...)