I do make icons sometimes (like the ones for this account and several of my rp accounts), but I wouldn't get upset if people "stole" them from me. Reason being, I do tend to just crop, resize, and adjust the color of things. Almost everyone can do that. If anything, I'd be flattered that someone likes what I made, with or without proper credit. On the other hand, I do try to credit people if I know who to credit. A lovely person over at [Unknown LJ tag] made an entire set of icons for one of my PBs because I requested it, so of course I gave them credit in my user info.
Also, I Google a lot of images to make icons, and a lot of it tends to be fan art as well. Since I don't know who to credit, I leave it uncredited. However, I also have a paragraph in my HMD posts that says if someone recognizes an image that I've used that belongs to them, to leave a comment so I can give them proper credit.
In short, it's not a big deal to me if I don't get credit, but I try to give other people credit when I can. Like you've said, someone might like what I have and want to know where I got it from (if it wasn't already mine in the first place).