Max Image Size Control is a very nice plugin. Thank you very much.
When I made custom sizes, however, it was a bit difficult to find the new files. It would be cool if they could show up in the Media Library, but if that's too difficult, maybe the files could be named picture_name_custom0.jpg, picture_name_custom1.jpg, etc. (That way it is easier to guess the name after resizing, than with the standard WP system of using the size.)
I don't know enough about programming to know what I'm asking, though, so it's nothing more than an uneducated idea.