You will need some basic GIMP skills, but (I find that) most of it is self explanatory.
Step 1: Take Your Picture.
This is my picture, of the Christchurch port hills.
Load it into the GIMP (File>>Open) and click Colo(u)rs>>Threshold. Slide the slider until you get a nice silhouette (This may take some tinkering!) my final silhouette is the second picture.
If your picture is a panorama (like mine), not a nice 4:3 ratio, I changed the canvas size (Image>>Canvas size) to be the right size, then click Layer>>Layer to image size.
If you have a widescreen, you may not want to do this..
Step 2: Add Layers
Make sure you have the layers dialog open (Windows>>Dockable dialogs>>Layers)
Create a new layer (by clicking the little piece of paper with a star in the corner), drag it to the bottom, call it 'base gradient'. Lets get the creating layers over and done with shall we? Create a new layer above base gradient called 'dark sky', and one on top of that called 'Glow'
Step 3: Add Gradient
Make a gradient (Click and drag) from the middle to the bottom so it looks like so:
Step 4: Add Dark Sky
Step 5: Add Sun Glow
It should look something like the picture below.
If you have 'holes' (like my bay) in your hill silhouette, keep on reading!
Right click on the 'Glow' layer (In the Layers dialog) and click 'Add layer mask' a dialog will pop up, click 'Add' (or OK/ Affirm/ Yes/ Roger!) the layer should have two little squares next to it, one is the layer, the other the mask. Select the silhouette layer, and press CTRL+C. Select the mask, and hit CTRL+V.
A new layer should pop up, right click it and 'Anchor layer'
Use the 'Paint' tool (Setting your foreground colour to black), select the mask and paint (click and drag) over the 'holes', as in the second picture.
Step 6: Add Sun
Create a supernova (Filters>>Light and shadow>>Supernova) click the colour button, and change it to orange (different to the 'glow') set the spokes to 0 and the random hue to about 0-10, set the radius to what ever you think is appropriate.
Use the move tool to move it to the prime position.