The "Enter URL" box on the front page has "http://example.com" as the hint, but if you type in "http://example.com" it adds a second "http://" to the front, i.e. "http://http://example.com" and then fails to load (the image is a 404).

You have to be sure to OMIT the http:// part for the site to work.

Also, on Chrome, dragging on the image frequently literally drags the image (rather than drawing). When the drawing does work, it happens about 2 inches to the right of where my cursor is actually placed.