-Open GIMP
-File > New
-Click the drop down Advanced Options and set the Fill to Foreground
-Click OK
-Make the canvas black
-Filters > Nature > Render > Flame
-Set the Zoom to 2.75
-Click Edit
-Set the Variation to Spherical
-Click one of the 9 boxes and if you don't like any of them click Randomize
-Click OK ((Time Consuming))
-Filters > Colors > Channel Mixer
-Change the Output Channel to whatever color you want to be the main color of the image
-Play with the rest of the colors from there and then Click OK and save your image


You'll end up with something similar to the image in the attachment. Fell free to post your results if you use this tutorial.