Pic.1 Open the Stencil Factory and select your image (gif, png, jpg).
"Convert & copy", as its name suggests, also copies the output in your clipboard, in the first version.
Pic.2 "Open in Customizer" and paste in "input".
Pic.3 "create STL", download and print.
Stencil Factory also available in .zip (with sources) for offline use.
Thanks to Tony Buser.
Flash version contains:
PotrAs - original code Potrace by Peter Selinger
Ported to AS3 by Nitoyon. With Keith Peters' components
Thanks to them.