They basically take the hi-rez photos from the gallery or forum posts and use thier computer to trace the image.
For my Mr. Bean, they probably just took the picture, brought it into GIMP. With a few filters, they can extract the pattern from the picture of my finished cutting. So with relative ease, they can steal my pattern without much effort. It may not be as good of quality as the original pattern, but they get pretty close.
For something like Clayton's bison intarsia, they'd take the finished picture and trace the image in a program like Inkscape. The problem with something like that is they're not stealing from Clayton. He didn't design the pattern. In fact, they're actually stealing from the person Clayton bought the pattern from.