A trick that I use in animation much of the time hasn't been mentioned here, but might save you from having to bother with Photoshop (a program which I don't enjoy because it's so very unintuitive).

Render your furthermost background element first. Save it as a jpeg (or a tif, tga -- whatever you prefer).

Then start a NEW scene and load that image back into Poser as a BACKGROUND image.  The menu selection is:

"File > Import >: Background Image"

Then arrange the next closer layer of new objects in FRONT of that background image.

Now save that combined image out also as a jpeg (tif, tga, whatever you prefer). 

Then start a NEW scene and load that previously rendered jpeg in as a background image for the next layer of new stuff.

Keep rendering out stills and load each previously rendered image into Poser as a background image for the next new scene.

If you build your image up this way, you can do the whole complicated thing in Poser without ever having to meddle with bucket sizes, texture resizing or any of those messy things (I've been using Poser for years and I have no idea of what a "bucket size" actually is, nor do I ever want to mess with it).

I even do this with animation and moving-camera shots, rendering background stuff first and then loading it back into Poser as background avi's.

You end up only rendering what your computer can handle, but you're not limited in any way in terms of the complexity of your image.

Hope this helps!

-jjsemp

www.creeporia.com
(animated with Poser)