What tools might be available in Ubuntu to scan many image files and detect similar ones despite different sizes, formats, etc?
Support for batch mode/cli would be especially good. For example, so I could write a script to favor the higher resolution/color depth versions and delete others.
Graphically Digikam can do that.
For the command line you can use
findimagedupes.The simple command to find recursively and report similar images is
findimagedupes -R ~/Pictures.