IconVault's Image Compare puts two images on one canvas with a draggable before/after slider, so you can sweep between versions and spot exactly what changed. Switch between split view and side-by-side, and get a live difference percentage computed from the pixels, with an adjustable threshold for how sensitive the comparison is.

Everything is processed in your browser with no upload and no signup, and it is free to use, which makes it safe for unreleased designs and client work. Honest note: the difference score is a raw pixel comparison, so compression artifacts or tiny shifts can inflate it; use your eyes for the final judgment.