We get a lot of traffic from Google images. What we have found is that the actual file names of the images themselves seem to be attractive to Google. We started using longer, yet more concise, file names some time ago. For adult, using a models name in the file name seems to work nicely, especially when using dashes. For example instead of image10.jpg we might use Nina-Hartley-black-lingerie-blowjob-001.jpg.
We started noticing this a couple of years ago as we were getting quite a bit of traffic from the images anyway. Once we started tweaking things like the file name our retentions seem to increase.
Also, we use a simple program called Fast Stone to rename our files in bulk before we put them on our sites. Great program for resizing/renaming files en masse.