I got an idea I want to map out, but having recently started playing around with the mapping tools, I see that the game won't allow background polygon creations. It's tempting me to just use a large source image and build over it to get the detail I want, but I don't believe I've ever seen that done. Is that because it'd wreck the game, be too large a download to get people to use it, or some other reason?

If you use a huge .png, then there's no problem. Bmp's might not load for older gfx cards, and might cause other errors too, along with general lag in-game.

well actually, in one of my recent maps i just put a big ol backgrround imige of a mountain and some snow. it didnt lag that bad and was small picture streched out.

I've seen this done in many maps. (The something-dawn maps by G_D).

Anyways, heed Demonics advice, stick to .png for large images. Actually stick to .png for everything (it's just better).