First make sure that the black on your transmap is TRULY black! if it's just the slightest it non-black it will show up as ghosted. In Poser 4 I usuallyset bot trans min and Trans max to 100%, I'm not sure if you really need to set the trans min to 100 too, but I do to be sure. Falloff should be 0. Highlight should be black and so should the reflex unless you plug your transmap into the reflection too. If you want to save your square with a transmap applied, you save that square to the props palette. An easy way to make a png is to render a 91x91 version of your transmapped square and then save that to the palette with the same name as the pp2. (I am assuming you have Poser 4 Pro Pack sinnce you mention png, if you have Poser 4 you could then use P3DOExplorer to make the png into an rsr.

FREEBIES! | My Gallery | My Store | My FB | Tumblr |
You just can't put the words "Poserites" and "happy" in the same sentence - didn't you know that? LaurieA
  Using Poser since 2002. Currently at Version 11.1 - Win 10.