You will need Gimp
and any aerial photograph.
if you're running windows, you'll also need a focus blur plugin
Gimp for linux has focus blur built in.
Step 1: Selecting a picture.
Step 2: Add a layer
In the Layers window Click new layer and name it whatever you want.
If you don't see a Layers window, press Control-L (or select it from the Dialogs menu)
Step 3: Add a gradient
Now click the Gradient tool in the Gimp window.
Next, click and hold near the bottom of the image and drag the mouse to the top of the image, a line will appear as you drag, it is important that you keep that line as straight as you can. Release the mouse, your image should now be black on the bottom and fade to white at the top and look like the (second) image below.
If it has White at the bottom and black on top, click Colors and select Invert.