[quote=@Jove]
The only policy I can imagine for that is for people to provide credit and sources for all of their pictures. But what's the point? The issue barely ever comes up, and if someone really wants to find those images they can use a reverse image search. It's not RP posts from years ago stealing money from online artists. They should spend the time they use hunting down "unauthorized" usages of their images and get a job if money is what they're concerned about.
[/quote]

My thoughts exactly. As long as someone isn't claiming credit for the image, or selling it for either real or virtual currency, and provides credit for the image, I don't see a problem. To make things safe, I personally ask artists if I can use their art for RPs (big shoutout to Hellraptor on dA for that, while I'm at it). However, if the artist says you can't use it you should be a good sport and not do it. Take the same policy Weird Al has for making parodies and apply it to RPs.