Ressources
This post will list various websites where you can find screenshots and fanarts for your icons. Feel free to contribute by posting links in the comments.
OFFICIAL ART
General creative uncut (concept art)
Final Fantasy FF XIV ARR (wallpapers)
Guild Wars GW 2 (wallpapers & concept art)
League of Legends wallpapers & concept art
Mass Effect ME 3 (concept art) | ME 3 (wallpapers) | The Art of Mass Effect (concept art)
SCREENSHOTS
General deadendthrills | gamerevolution | VGIMG | The Video Game Museum
By fandom Final Fantasy XIV ARR | Mass Effect 1 - 2 - 3 | Super Smash Bros. 1 - 2
FANARTS /!\ Don't forget to credit the artists and put a link to the original art when you use fanarts. On zerochan, the artist (and where the art comes from) is often mentioned in the tags so you can track them starting from that.
deviantart | zerochan | minitokyo | pixiv | danbooru
USEFUL TOOL
The website danbooru references very well many fan artists, their links will be in the "artist" tab (check that they work, some links are dead). You can use that when you find a fanart on danbooru (the artist is in the tags, click it, then you just have to go on the "artist" tab) but also when you find a one on zerochan for example. On zerochan, the name of the artist is given in the tags, but not the link to their personal page, so you can just search them on danbooru, that will give you the links you need.
thanks to iconsoleander and darkpalace for contributing