I learned this in 3D Commune when it had many many masters of Bryce, that when you are in the (Terrain editor)  and you have clicked the (Pictures) tab in the (Editing Tools) panel and then have clicked (Copy} your picture will be in the clip board.  You can then open your pict in photoshop and after you have applied a filter or etc., and saved it as a bmp.You can then  import it to Bryce as follows...Open the Terrain editor...In the (Editing Tools) panel, click the button on the left  (Picture. note. this is not the Pictures under the tab Editing tools!) locate the file you have just saved and click it.  You will then have a smooth and clean terrain. I work on a Mac. and my software is Bryce 6.1 and PS C3.