Ok render your scene in Bryce, then making sure all the objects in your scene are selected choose object mask form the file dropdown menu and click render. You should now have something as you see above. In photoshop open a new fileand drag the original bryce render and the black and white version on as a layer so that both match each other. Then open your plug in file background you want to use and drag that on as well. Select the object mask layer and using the magic wand tool select the black areas. Switch to the original Bryce render layer and use ctrl x to cut out the areas you don't want. Discard the object mask layer and you should get the result you want

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