I attached my method using GIMP and Inkscape. It takes some practice - the best way is probably to look for others to post a request for a pattern, then try to render a pattern from their photo. That gives you a chance to compare your results to what others did with it. A part of the work is deciding what to actually convert - it's often better to crop the photo to a smaller image that highlights a face, or to bring 2 faces closer together for the pattern.
I hope the attachment holds - they have been hit-or-miss lately.