Am I the only person who really hate it when people use fanart for icons without crediting, and then ask people who use the icons to credit them? Personally, I don't use fanart for icons, unless I draw it myself or get permission from the fanartist, but I can tolerate at least if people credit properly.
Using official art is not without legal issues (which is widely ignored in the fandom as long as no profits are made) but at least it's recognizable as official art, which people know who to attribute to. Also the official artists were already paid for their work.
Fanartists don't get paid, they draw something because they want to, and they display it to other people because they want to be appreciated (otherwise they'll just be happy with drawing it and keeping it to themselves). The same thing iconmakers do. So using someone's work to gain appreciation/fame without sharing that appreciation/fame to people who does more work than you (yes, drawing fanart is more work than editing icons, I know because I have done both) feels like there's a line crossed. I'm not even touching the issue of permission or cropping the artist's name when it's written on the pic itself.