You can use the standalone File Compare/Merge to compare two images on your computer. You cannot edit the files directly, but you can swap the contents of one image with the contents of another one.
File Compare/Merge provides a toolbar with a few common actions that control the editor pane display. It also includes some image comparison tools for zooming in and out, and moving the file around.
The content of the source image replaces the contents of the target image without change the filename.