In my recent answer to a question I've used rather a lot of images from BrickLink. Is there a need to get in touch with BrickLink and the other main LEGO sites for permission to use their images.
I know that submissions to this site is made under a Creative Commons license but I don't know how that affects stuff from other domains, and I expect the sites can take the bandwidth hit, but shoudln't we ask out of common courtesy? Also a lot of them get re-uploaded to imgur through the SE uploader.