Before: the Diff module should marked changed images. But the Diff module always marked every uploaded image as changed (the links were always different, even if the images were exactly the same).
Now: only links to images are compared, but not the binary content of images themselves. So image isn't marked as changed, if has still the same path, even if his binary content is different.
Related issue: http://jira.magnolia-cms.com/browse/MGNLDIFF-21