diff will help you find duplicate files in two different directories, but if your mess is greater or if, for any other reason, you want to find duplicate (exact) image files in a whole directory, including subdirectories, you can use the gthumb image browser viewer, which is probably already installed in your system.

Gthumb provides a tool to search for duplicate media/audio/video/images/text/all files in a directory. To do this, just select your directory in the view mode that displays a left pane with your directory tree, and then, from the menu select Edit>Find duplicates... a dialogue window shows the duplicates and lets you choose which file(s) to delete. This procedure is visual and helpful in many cases; but it is slow, if you have too many duplicate files to delete.