Note that for the cover her dress changes, his suit changes, the chandelier, electric lights, etc. are gone.
Notice that the face changes, the dress changes, her hair changes and that in the original, the background stops at the top of her head and the back cover is from the part of the photo not used on the front cover so the effect is a wrap around.
The good thing about GIMP is that the more one uses it, the more one discovers it's possible to do with it. There are also youtube videos online that explain how to do things--though it didn't always seem to work quite the same for me. But for free software, GIMP is amazing.