How can I do to make a watermark printed on foto in the same proportio

How can I do to make a watermark printed on foto in the same proportion? At this point, a watermark in large photographs ranks as a small point, but in small closes half of photo.

http://www.boonex.com/m/Watermark_Plus_for_ImageMagick

I think that's what I need. By the description of the module is excellent. I'll try.

Thank you!