Here's kind of a step by step. I still use PS 5.5, should be the same as 6 Open all 3 of your images, The render from Bryce, The Mask render ( from Bryce ) and the background image from Universe creator. For best they should all be the same resolution. Select the Move tool, then hold the shift key and drag and drop the Bryce render then the Bryce mask render to the Background image. If they are the same resolution all 3 will be centered to the canvas. If your not familiar with layers in PS, you will need to open the layer tools, Window/Show Layers. all 3 images will be separate layers. Click the mask layer making it current ( it should also be the top layer ) then do as Claymor said and use the magic wand to select all the white areas in the mask ( you may need to shift click with the wand tool, to make multi selected areas. Once the selection is made, hide the Mask layer by clicking it's eye icon in the layer window. Then make the Bryce render the current layer. Right click the layer and select Layer Via Copy. This will create a new layer of just the selected area. Then discard the old layer. You may want or need to use some of the tools found under Select/Modify to adjust the selection before you layer via copy. You can also save the selection under Select/Save Selection, so you can call upon it at a later time ( Select/Load Selection ) without using the mask again.

Stephen Ray