Need fast help

[Zorba]

A simple way would be to just have the two images as separate layers, then cut the top one diagonally from corner to corner. So open your first image in gimp, bring in the second as a layer on top (like 'file'-'open as layers' or pasting it into a new layer). Then go to the top layer and cut out the triangle where you want the bottom image to come through (use select tool from corner to corner, then around the rest of the half, then 'cut').