In my early days of using GNU/Linux full time, we had an application called Gwenview. It was, and still is an excellent thumbnail viewer. But back then it also used to have an excellent set of plugins called KIPI. Sadly the really useful ones such as resizing and changing format completely disappeared when KDE 5 came along.
Alessandro Francesconi’s BIMP plugin for GIMP is excellent for doing Gimpy things en-batch. It also allows sequences of multiple transformations which you can save as *.BIMP files for reuse later. I find it an astonishingly useful tool, especially since KDE screwed-up Gwenview’s KIPI plugins…