In the past I went with "credit spell/characterisation in the author's notes" but in this case, the idea I needed was a bit too vague and huge to be that easily credited. I ended up asking the author and she gladly gave persmission.

I make icons myself and it annoys me when other people take them and claim them to be their own. I know they are tiny and I didn't take the original picture but sometimes, especially if I made the screencaps or animation myself or if it took hours of googling to find the picture I wanted, I want to be credited.

Also, as some others said, I like going through people's user icons and if I find one that I like, I'll take it and then check out the person's icon journal or community and usually if there's one good icon, there tend to be many more. :)

Then again, I'm a total icon whore, so I might be a special case.