>I am not cropping, I am removing specific parts of images (let's say a
>triangle) i.e. turning it transparent. The specific parts needs to be
>removed are same for all images.
>I could do it manually for each image, but there are just too many of
>them (was a video converted to frames)
Is that area static, if it varies then I think not possible.
Nothing easy in Gimp I can think of to batch process a set of frames. You will
need a script/plugin writing.
As a possibility, using ImageMagick and a couple of commands.
overlay each frame with a mask, choose a colour that does not normally occur in
the frames, say magenta