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I wasn't sure if I should disallow indexing in the subdomain's robots.txt file. I hate to loose any image referrals, but it doesn't make sense to just give access to images with no other content. Any suggestions?
In fact, Google Image Search seems to try to accommodate the practice too, because they will often credit an image to a site that hotlinks to it, rather than the actual owner's site.
I've never seen a negative impact on the ranking of pages because of it.
No negative impact for the page if all the attributes are completed correctly however there can definitely sometimes be a very big impact for image only search rankings in Google.