I want to make a backdrop using building pictures. I want to however make them appear different distances from the background scene. How do you go about doing it. The flats will be attached to the background scene.
To achieve that you'll have to use a trick called "forced perspective". The backdrop should consist of at least three layers which are placed in front of each other. The first layer should show large-scale buildings, the second layer should show medium scale buildings and the last layer small scale buildings. The colours should become lighter with any layer. A sunny sky with clouds would be a nice finish.
The different foreground elements can be attached to the backs with pads which increases the 3D effect. One of the best things of such a backdrop is that the perspective changes with the position of the viewer, similar to the old showboxes.
Of course you can make a simple flat 2D backdrop using the same techniques described above: several elements in different scales stacked on top of each other. This can be made easily in Gimp because it supports layers. You can move the elements around and re-scale them till they meet your expectations.
I have attached a simple example of a 3D backdrop which can be used as a guide for making a 2D backdrop. (If you want to make it 2D just create the different layers and bake them together in Gimp.) The last pic is an animated *.gif which shows the 3D illusion when the viewer changes his / her position.