To prevent this I would strongly recommend, and be an advocate for, an "by artist" field @The Dark Wizard . If we can implement a sitewide "by artist" field to any images uploaded, including profile images, I believe many people would share the same sentiment that @peritwinkle and many others share.
While we do customize the site quite a bit, we are ultimately using a forum software that we didn't create to power the site. There is almost no way to logically implement this for avatars, character sheets, or for the site to even notice the difference between a hot-linked image, or uploaded image for the purposes of this request. Anything custom we make will most likely break once the forum software updates, assuming we could even figure out how to do this. I could add this to the Gallery much easier but the vast majority of images on the site are not using the site's gallery feature, opting directly to interact with the post. There's also no feasible way to force everyone to upload to the gallery first and then link images from there to posts. (This is one of the few painless ways I can see of enforcing a by artist field site-wide).
I always try to credit artists when I can find them and have built it into most of my bbcodes. While logistically a nightmare, I can definitely say I'll bring it up with the rest of the staff. In the meantime we do try our best to honor all take-down requests we receive from artists who link to the image from an official portfolio (like deviant art ond so forth). We've never denied such a request since we believe artists have rights on RpN.
Edit: I suppose that with the RP Creator rules, a RP Creator could set a rule where they will reject your character/picture if you don't credit the artist.