I sign mine in lower left corner,,,,because the FAA watermarc overrides it if is in the right corner, I keep a sig within Preview on the IMAC and just add it when I upload, I can change the color to match the painting, Try to use a font style that is signature style ,,,I can actually create my sig on preview within iPhoto but my signature basically "sucks,,,so I use a font that is signature style,,,see example