make the image, set its path to whatever .blp, set its type to (doh, i forget which one, i know that some of them don't work for some reason, and occlusion makes the thing all dark) but besides that set its always render to true and thats about it
o, ps, size is in war3 grid settings, so 128 size is the size of 1 small grid block, 1024 is the size of 1 big grid block, and run trigger show image last created image at the end
i wish images alpha'd out the borders when u don't use a side-transparent image though