Contains a fair bit of theory too for the interested. I used to run a fractal webpage and created numerous Mandelbrot & procedurally generated 3D landscape programs in C and assembly.
A good program to play around with for making images is XaoS, which was around in the 90s. It should absolutely fly on a modern PC! Looks like something David Bowie might have used http://wmi.math.u-szeged.hu/xaos/doku.php