Need to convert 10, or 100, or 1,000, or 1,000,000 images? You might use GIMP for the first. You'd best give it up for the 2nd and further.
On Linux desktop, I had an Imagemagick command in my command history which would trim off the top n pixels of images to remove extraneous elements. Quick to call that up and run it on a new screenshot, or a series of shots, prior to posting online. The job was completed before GIMP would finish initialising.
So can the Gimp. I wrote a calendar script in Perl for the Gimp literally 15 years ago.
Imagemagick though makes simple-to-moderately-complex stuff bog easy.