My favorite is http://daisypath.com/Vacation_step1.php

It has to be the BB code. All of these sites have a BB code option to copy and paste. Like Rob just said; You can't use the HTML option.

It worked best for me when I deleted the URL code from the front and back
Be sure to keep the [img] and [img/] in the beginning and end.

Then it will show in your signature.