Once google indexes them I don't believe you can alter their use to your advantage such as adding popunders, redirects, or links to buy pages, without adversly affecting the original usage of the image.

Watermarking is about the only way I can think of... well i suppose you could do a .htaccess anti hotlinking redirect on them for that subdomain, but it would not take long for big G to see what you are doing and likely cause other residual damage to other domains that have been indexed in association with those images