Now I know a lot of us just find images, save them and don't say where we find them, too much work, right? With general images, that's not that bad. With avatars/icons...some people would appreciate if you did credit them. My main problem with noticing the crediting is in Fan Art.
Fan Art isn't just a photo or picture created by someone for you to just take and claim as your own. It's something someone has created themselves. A lot of people have to put hideous water marks on their work after a while if people see it as a grab and take situation.
All it takes is when you find the picture add it straight to Fanpop and have the page still up of who it belongs to. Write their name/screen name and what site it's from and this can clear up a lot of awkward questions of "did you make this?" and unhappiness of people finding their work uncredited on other sites.
Just something you should think about when you post Fan Art next time. Thanks :)