Your response seemed to imply that to make the PDFs work I needed to define them as "multimedia" files rather than as "image" files in the Photopost configuration area. I say that because that's the only case where I get prompted for a thumbnail name.
I changed that and now my PDFs upload okay but show up as "multimedia" files using the multimedia icon rather than some sort of PDF icon. But at least the program no longer complains about an inablility to create a thumbnail for the PDF.
But when I try to use another file as the thumbnail for the PDF when uploading, there IS no thumbnail shown for the PDF in the 4 col x 3 row image gallery. Instead, I get a red "x" where the thumbnail should be. However, when I click that "red x" in the gallery display and it shows me the "mid-sized" image, that image is the one I selected as my "optional" thumbnail.
Is that the way it's intended to work? Or is there some way to change that?
Is there an easy way to get the PDF to use it's own ICON type like the multimedia does?