If you do intend to use this as a background (and please do, you have permission), your image should be big enough to fill an entire screen without looking pixelated or stretched. 800×600 and up works great for a Powerpoint, but gigantic isn't good either. For the purpose of saving a little space, my screenshots will be of a 400×300 image.
1.) Plasma was used in both the lighthouse and heffalump tutorials.
2.) In order to get smooth contour lines, we need to start by blurring all of the colors together pretty intensely, using a Guassian blur.
3.) This next tool is what defines the contour lines. GIMP documentation defines the Posterize tool as "designed to intelligently weigh the pixel colors of the selection or active layer and reduce the number of colors while maintaining a semblance of the original image characteristics." Basically, it separates the image into a number of color ranges, and assigns each range a color representative of it. The screenshots say all.
4.) Now we have the contours, and you might actually stop here. However, to me that looks like a mess. Again quoting GIMP documentation, "The Colorize tool renders the active layer or selection into a greyscale image seen through a colored glass."
5.) I found two nice ways to finish this image, if you're not already satisfied. First is a bump map. What that does is it takes one layer, converts it to a grayscale 'map', uses the lightness of each pixel as a 'height' value, and shades another layer to make it look like each pixel actually has the height assigned to it. If the first layer has hard edges, like in this case, you effectively get nice 3-d edges on the second layer. For this image, the first and second layer will be the same so that the edges match up.
You might find that the edges become a little pixelated after the plugin executes. The final image (last link) is Guassian-blurred with a radius of 2 pixels, and that seems to help.
6.) The other way is very similar, and actually uses a namesake of the bump map plugin from step 5. Make sure that bumpmap is checked instead of emboss, and then play around with the sliders to get an effect that you like. This method doesn't seem to have the pixelation problems of the previous one.