Step 1: Adding Clouds
Once you have selected an image of a house, open it in GIMP by selected File>Open. Once that is done, find your cloud brush under the "Brushes" tab. Or, if you are new to gimp, go to deviantart.com and search under the GIMP brushes tab and find some you like. While you're there, you may consider looking for chain brushes, blood brushes, grass brushes, and tree brushes.
Once you have your brush ready, make the size of the brush about 1.00-1.50 and put the clouds all around, from the bottom up, but leaving some of the house visible.
I made the clouds look like fog, so I made the color gray, but be creative!