I created a terrain in Bryce, exported the grayscale map as a tiff, opened it in Photoshop, manipulated it, saved it, and applied it to my terrain back in Bryce.  The resulting terrain renders all jagged and stairstepped compared to the original terrain.  Why is that, and what can I do about it?

You can use the controls at the side of the terrain editor to smooth/raise/lower/manipulate your imported terrain.

Maybe before and after tiff files can make other member give better solution.

 Yes indeed..post your image in the thread...well a .jpg as a tiff won't post, so we can better assess the problem.. ...

sounds like photoshop saved it as only 8 bits per channel, where bryce terrains are 16 bits per channel - the 'stairstepping' is a dead give-away.

I think calyxa is spot on here.

 With Bryce 6.1 you can export 16-bit TIFF and PNG formats but you also need Photoshop CS3 to load and modify it. CS2 will load 16-bit but there are only a very few CS2 tools that work with them. Even CS3 doesn't support the whole tool set in 16-bit.
There are some problems with 16-bit PNG export from Bryce  Mac - can't remember about the PC.

Actually, the problem is more fundamental than I initially realized.  I created a terrain with planetary scale, and rendered it...

then I saved the grayscale map as png (I also tried this with tiff).  Then immediately reloaded the map back to the terrain editor, and got the stairsteps.

 Just to be sure, you're using 6.1.13.1? Find this by selecting the Help>About Bryce menu item while holding down the Control (PC)/Command (Mac). Here's a test I just did with PNG (also did TIFF). Left is the original, right is the image import. I used the down arrow pop-up at the top of the Terrain Canvas window and selected the Export Image item to export the image. I'm also at home with only a Mac, so this may be a PC issue.

I just run Paintshop along with Bryce and copy/paste from terrain editors' picture window to PS and back.
 Now and then terrain editor won't accept the paste,I save from PS as .bmp and that seems to work fine! 
Saves the fuss of exporting and is much quicker!Suppose this works with Photoshop too!

  Cheers! Jed. 

 Thanks Jed - didn't realize the clipboard did 16-bit copies.

 No idea about 16-bit stuff?,seems to work for me!Never tred it for 'planetary size' too big and slow to work with!....works fine for more regular sizes.
 
 Cheers! Jed.

 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.