Not legally, I suppose. Unless they come with a specific license that grants usage for non-commercial reasons.

My guess is.. NO.
Sorry.

If it's a game for sale, no (I realize you specifiy non-commercial, but just want to cover the bases here).

If it's for personal use and not being distributed, then strictly speaking, probably not, but you're not likely to get in any kind of trouble over it.

Pretty sure you can do whatever you want in a non-commercial game as long as it's truly non-commercial. Remember that just because you don't charge for something, it doesn't mean it's not commercial.

Recently, a professional development studio released a freeware FF7 remake in beat'em up format, which was quickly shut down by Square Enix, for obvious reasons that go beyond the notion of protecting their IP. By using the FF7 IP, this studio was in fact promoting themselves, even if they didn't make money directly from it, thus it's obviously foul play. There are, however, numerous rom-hacking demakes of FF7 for old consoles which Square Enix would never have a reason to take action against because they are truly non-commercial.

Thanks for the feedback.

All I'm doing is laying out what I call a "medieval combat simulator" (that could grow in a full blown RPG) and I thought that I could use the avatars as PC/NPC icons.

The game (well.... a few lines of code) are on github.

Nothing 3D chaps... ;)

A better idea might be using freely-offered stock images from somewhere, or maybe finding some good images on a site like deviantArt and asking the artist(s)/photographer(s) for permission to use them in your project.

What he said.

Check out OpenGameArt, they have loads of free to use art for games, most of it quite good too.