Kirk, I hate to say it but I have this problem all the time in OS 10.4.9 with PM 4.4.3.3, it happened on Tuesday actually. Copied two thumbnails to the desktop and only one was visible. I had to open the Desktop folder in a Finder window, move the "invisible" photo to a different folder and then move it back to the Desktop to make it show up.
I've long assumed that this is a problem with the OS. On that note, is there a way to force a refresh/redraw of a current Finder window or the desktop?