...not permanently watermark original images?
 

I don't want to permanently watermark the original images, because I may want to change the watermark.

However, the original images open directly, without going through a php file, so they don't get watermarked.

Is there a way to do this, or am I going to need to write a hack?

Here's the hack I wrote to do it
http://www.photopost.com/forum/showthread.php?t=121255