How to make a pattern
The easiest way is to take pictures of leaves or bricks or some other stuff. The
more chaotic the image, the better. Also, the image has to be small. One thing that
I do sometime is make a picture of a forest, take out of it just the trees branches
and leaves and shrink the image a bit.
Anyway, I haven’t experimented that much with patterns. Also not all patterns will
work with a certain mask. If the generated stereogram is messy, try with a new pattern.
Uploading a picture of your self small will work in some cases, but it will mess
a bit the image. But most of the time the hidden image can be seen, although not
that clear enough.
Anyway, enjoy, experiment with patterns, have fun!