GIMP :: Find Changes Between Images?

I have two satellite images that I am comparing to look for changes in topography between the two dates at which the images were taken. I was going to use the Layers Divide Mode, but the problem is that any brightness changes AND non-changes show up as white (the algorithm gimp uses is 256*Image1/(image2+1) ), so you can see if the pixels of image1 and 2 are the same, it'll give a pixel with value 256, and if image 1 has brighter pixel than image 2, it will give a value greater than 256...which gets converted back to 256. I've also looked into the Difference and Subtract modes, but they take absolute values or sets anything negative to 0, which skews my results. Basically, I'm wondering if there is something in gimp which will allow me to compare the two images, kind of like creating a ratio image in ENVI where darker colors stand for darkening over time, whiter colors stand for brightening, and greyish colors show non-changes. I want to divide one pixel value by the other, and use that value to create a new ratio image.