There are two things you could try. The first is GIMP scripting or GIMP BATCH. These are ways of running GIMP in BATCH mode and scripting actions to be performed on a set of images.
The second way, which is much easier and, in your case, probably sufficient is the wonderful ImageMagick suite. You can probably do everything you want using the
convert tool of that suite. The following are some relevant options, you can see the whole list using
man convert or by visiting this page.
convert - convert between image formats as well as resize an image, blur,
crop, despeckle, dither, draw on, flip, join, re-sample, and much more.
convert [input-option] input-file [output-option] output-file
-background color background color
-crop geometry cut out a rectangular region of the image
-trim trim image edges
-size geometry width and height of image