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.
Some links:
http://en.wikipedia.org/wiki/Forced_perspective
https://www.google.com/search?q=for...wX6xIHIBw&ved=0CD8QsAQ&biw=1280&bih=643&dpr=1
I hope this helps! :thumb: