These are not fractals! Each image that you see is a map of the probability that a point (x,y) lies on the pixel canvas, and the points are calculated as follows:
x' = sin(a * y) - cos(b * x)
y' = sin(c * x) - cos(d * y)
These might best be referred to as Peter de Jong Attractors.
The program used to render these images is Fyre, written by Micah Dowty and David Trowbridge. It is open-source and available for use on many platforms. It can be found at navi.cx or by accessing this fyre.navi.cx.
If you have a picture that was made with the help of Fyre but isn't listed in my Favourites, then please send me a link so that I can include it for