I got a trick to help soften pictures and add extra cool.
(I don't know GIMP. This is for PhotoShop, but GIMP might be able to duplicate the effect.)
Duplicate the main image in a new layer. Go to town with brightness and contrast until the image is mostly black with 15-20% splotches of light. Gaussian blur that layer, in this case maybe 15-20 pixel's worth. Now screen that layer over the original layer. This adds a "bloom" effect you will like. (You should probably lower the opacity of that layer below 100%, too.)