Paste PICT as BG Tilemap

How does "Paste PICT as BG Tilemap" work?
I am working on a huge underground level (400x100!), and it'd be a real pain to draw BG tiles by hand.

------------------
"Give me a large enough hard drive, a fast enough modem, and I can download the web!"

I think it's the same as Paste as FG tilemap. I haven't tried it, though.

------------------
Stupidity got us into this mess, why can't it get us out, too.