I finally found the cause of "upxp64.dll" as an autostart item. It's during installation of a little shareware utility program called "Photo Stamp Remover", the purpose of which is to remove watermarks, date stamps and other unwanted details/items from your digital photos. It actually works quite well and is very handy, I must say. Google for it, and you'll also find their nicely done online demo.
I cannot find any reason for this "upxp64.dll", though, since the program works as before even after deleting that earlier mentioned folder\file and registry entry. Odd.
Anyway, I consider this subject hereby solved.