Open Gimp, paste your image and move it wherever you like.
Layer>>New Layer
Layer>>Transparency>>Color to Alpha
Layer>>Transparency>>Alpha to Selection
Layer>>New Layer

Select>>Grow>>3>>Ok

Now go to Gimp and click on the bucket tool
Make sure your foreground colour is white
Click Fill Whole Selection [located under Patterns]

Now repediatly click on your image until it is filled with white.

Filters>>Blur>>Tileable Blur>>9>>Ok

You image may have some of your background colour left inside of it, now???s the time to get rid of that. Take your brush tool and make sure all the inside is white.

Hit Ctrl+L
Click on the second layer [should say Pasted Layer]
Click the up arrow which is located at the bottom of the layers box.
Layer>>New Layer

And now you should have a pretty white glow =]