Thats because if you use the watermark option when you right click you'll get the url WITH the watermark in the url, If you don't use watermarking you won't, you'll end up getting the url to the image..
NON-Watermarked
yoursite.com/photopost/data/519/medium/DSCF0304.JPG
Watermarked
yoursite.com/photopost/watermark.php?file=155
You could always turn on the option for showing the UBB code.
Admin --> Upload photo options --> Show UBBCode on Photo page? = yes