Here's a nice easy way to get natural-looking coastline shapes:
It's just alternating two steps:
(1) set the value of each pixel to the median value of a short random walk from the current point
(2) zoom in a little
Plus a little fine-tuning that adds some random black pixels if the center of the image has too many white pixels, and vice-versa, so the image stays "interesting" as it continues to zoom in.