Welcome to the Q&A Forum

Are you seeing the images getting indexed, though? Even if GWT recognize the Robots.txt directives, blocking the pages may essentially keep the images from having any ranking value. Like Matt, I'm not sure this will work in practice. Another option would be to create an alternate path to just the images, like an HTML sitemap with just links to those images and decent anchor text. The ranking power still wouldn't be great (you'd have a lot of links on this page, most likely), but it would at least kick the crawlers a bit.