I never liked the Construct, so I set up my Startup Scene to use the Poser 10 Workspace.pz3. Then just save your renders in .png format, and you'll always have a transparent background.

You can find it at Libraries > Scene > Poser 11 Environments > Poser 10 Workspace

Then just go into Edit > General Preferences and set Launch Behavior to Launch to Preferred Scene and click the Set Preferred Scene button. Then go to the Interface Tab and set Launch Behavior to Launch to previous state. Click the OK button at the bottom right to save your preferences. Then save a scene called Startup, and then close P12 and relaunch it to test that the setup worked.

Then you'll never have to worry about the unwanted backgrounds again.

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OK . . . Where's my chocolate?

Butterfly Dezignz