Using GIMP.
Filters > Map > Bump Map

For this image, my new image (canvas) was transparent, I selected a spherical shape with antialiasing/Feather edges 30.9 radius and filled it with my favorite color/Mode: Normal/Opacity: 100%/Gradient: FG to Transparent by picking a random spot on the left and pulling the line to the right.
I then added the bump map as shown below:
Almost forgot, the Bump Map used was an image I already had that I had copied a selection from to the clipboard.



