Google has made an update to the Google Similar Images filter to make it more comprehensive and fresher. They also enabled it to allow you to restrict the similar images to a specific site by using the site command.
Jay David from the Google Search team said in a Google Web Search Help thread, "today we're announcing an update to Similar Images, the image search feature that allows you to find images that are visually similar to a particular result. "
Jay said the feature is now "a lot more fresh and comprehensive" and also made "it possible to find similar images when using the site: operator."
To use the similar images feature, search Google Images and mouse over an image. Then click on the similar link. To then restrict it to a site, use site:www.domain.com with this filter option.
Here is a picture:
This might be a good way to find people who steal your images and how often they use it.
Forum discussion at Google Web Search Help.