ok well first thing I do is I open a new layer. go to the tab on the top that says "Layer" and click add ner layer. This will come in handy later.
ok now on the top panel thingy go to Filters. This is where you'll do all the amazing stuff. Lets start with Light and Shadow. There should be a Supernova option there (I have Gimp 2,6 so I dont know about older versions). Click on your new layer (you can use the "back ground layer" aka your original image, but then you wont be able to move it freely, you'll have to set where you want it to go) then click on the supernova option. here you should see a preview of supernova and the new layer. edit the settings (color, random hue, and radius size) to your liking. then click ok.
you should have a supernova then on your image. click the Move tool (blue crossroads looking thing) on your Tool box (on the left). Now when moving layers make sure you move the right layer (in this case the one with the supernova). Move it where ever you want.
**note** I use Fireworks before I use Gimp to make images fade and blend as well as alter colors and stuff. you cant do that in Gimp. Also I use it to arange the images. Its hard to do that on Gimp.