ImageMagick can do batch image operations.
It's a command line program, so most of the work will be in reading the
manual and searching to find the right command to do what you need.
ImageMagick is actually a suite of a few different programs. The one for
performing bulk operations on many files is
The example below removes transparency (resulting in a white background)
and resizes all .jpg files in the current location to 300 × 300
mogrify -flatten -resize 300x300 *.jpg
You will need to read the
mogrify documentation to
come up with the exact options you need to use for what you are trying to
Edit: Since you mentioned using GIMP, I also wanted to point out that GIMP has a batch processing mode. I've personally never used this feature, but it may be useful if you have original files stored as .xcf files.