I just spoke with Bing and this is what they said about removing images from their Bing Image Search index:
disallowing files or directories in robots.txt only prevents crawl, not indexation. If you want our system to organically remove these images, you should allow crawl and disallow indexation using a NOINDEX X-Robots-Tag HTTP header.
Several ways of accomplishing this. Since my server is Apache and my images are in several specific directories, I add this to the htaccess in each of those directories: