Here is my example of a post nuclear war White House for the Alternative History Image assignment. I began with this photo:
First I separated the White House from the background in Photoshop. I used the sponge tool’s feature to bring out the shadows of an object to give it that gloomy look. Next, I make a transparent brown rectangle over all of the grass. I used an eraser to erase the rectangle over the fountain and the path. After that I used a gradient on the sky to give it a post-apocalyptic hue. I’m not happy with how the sky came out, but I’m not sure how to make it look any different. Finally I merged all the layers back together and got the finished product.