VS Unused Images Finder
This Visual Studio Extension searches for unused images in your web project (and optionally removes them)
Download it from the "downloads" section.
OR - find it in the VS extensions gallery right within visual studio. Just type "unused image" into the search box.
Right-click your project in solution explorer and choose "Find unused images"
First, it scans a project for image files, then it searches for references in .aspx/.ashx/.cs/.css/.js files.
This is the early version. It works. But might have some UI-bugs...