I found what I was doing wrong, have a workaround to my problem (see previous post "Object Not Compatible") and figured out a couple of things about Home Designer / Create Plant Image, and also about the image software I was using (Irfanview). First, Create Plant Image wants to use the Alpha Channel option for PNG transparency, and apparently can't use transparency based on a particular color in the PNG palette. Secondly, I had been opening the files in the Referenced Files .zips using Irfanview (so I could view before saving), and then saving from there using Save As. Apparently, Irfanview has some kind of limitation or bug when handling Alpha Channel transparencies, and that's why I was getting a "ghost" Camellia when doing that. When I simply copied from the .zip file to a temporary location, and then used that file to do Create Plant Image, everything worked as expected.
I answered my own question, or rather I found a workaround (it still seems like some kind of bug that an object shipped with the product would be "Not Compatible"). See the other post I put just after this one "Create Plant Image".