Welcome to WebmasterWorld Guest from 188.8.131.52
Also, I've noticed Google has changed the way they display the images lately. It used to be that if someone clicked on an image in the SERPs it would open a new window with the image at the top and the hosting web page at the bottom. Now clicking the SERP images just opens the image and the web pages is not displayed.
So again, please explain what kind of benefit you think you are getting? I personally have my images folder banned by robots.txt but might consider changing that if I could see a benefit.