To clarify, say I have an image that is 100px wide and 50px high and I want to make it 50px x 50px, by removing columns from the MIDDLE of the image. i.e. I want to keep the left side of the image, and keep the right side, but remove the middle. I do not want to scale the image in any way. The image is a PNG, with transparent areas.
I'm not very used to GIMP, so be gentle :)