A couple other tidbits. If you don't want to do a secondary click and select "Add" from the menu, you can drag and drop the image file into the Cover Window in the Tag Panel, or you can copy an image and then paste it in the Cover Window (either by menu or keyboard shortcuts like any other copy and paste in macOS). You can also highlight existing cover and use Delete to remove just that cover from the selected files (you can have multiple covers on each file, if you so choose).
I would also recommend checking out this post and the other documentation for Mp3tag as there are a lot of powerful capabilities and shortcuts if you spend some time to learn how to use them little by little. Basics and Overview