The GIMP is great fun! Here are a few images that I've made with it.
The first two are distortions of me, made with the domain coloring technique. The functions used are w = z + 1 / z and w = sin z.
Here are some more or less abstract pictures, where I used the MathMap plugin to render something to start with, and then played around with standard GIMP tools like bump-maps, gradients, and layer modes. Click on the thumbnails to see the full-size images.
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 2.5 License.