How do I just drag & drop photos that I wanna sync from iTunes? Everytime I wanna add a new photo, it just deletes all the other ones from my photo library and leaves the one I wanted to add...?

I like to copy the photos that I wanna sync, to my desktop so it's easier for me, but when I do that it erases all the other pictures I had on the iPod to begin with.... How do I add photos to already existing photo library's?

Thanks!