(From memory) You get the PNG message only if you try to go from P3dO to Poser via PRPC, because P3dO associates each library item with its thumbnail - so this method tries to send both at once, and PRPC has no mechanism for loading a PNG into Poser.

Use the inbuilt PoserPython in P3dO instead. It's in the right click menu for any library item. You still need to start up the receiver script in Poser, of course.