ImageDiff does not depend on DumpRenderTree but for legacy reasons it lives inside the DRT folder, even though it is needed when running pixel tests for WK2. Some ports like Qt and EFL have the option to build only WebKit2 and it turns to be a problem for directory oriented buildsystems. We should move it to the Tools root directory and follow the same directory structure as we have for the MiniBrowser (Tools/ImageDiff/[port]/).
It is in Tools' root long time ago.