One thing I need to know about Poser, because I need to know too. What size does a sky only image need to be to be a decent image for backdrops and such within Poser? I know the angle needs to be up a little to get more sky. Should I render different angles of same sky? When I'm done I'll post a bunch in the free stuff area..

I'd probably make them 1024 x 768. Different horizon heights would be useful, perhaps up to about half way up the frame.

That's it? They don't need to be any bigger?

I haven't found a noticeable improvement using enormous backgrounds, if anything it slows things down and uses more memory. Some people might want to render a very high resolution image, and they would benefit from having more pixels, but for freestuff you have to think of the needs of the many; and download time is definitely going to be part of the equation. I suggested 1024 x 768 because it's my screen resolution; it's the size I usually render at; and I usually make my backdrops that size.