You can't. Silly, eh? As a workaround, take a copy of your PCF file and rename the extension to .ZIP. Then, double-click on the file. You'll see a "bitmaps" directory and (somewhere) in there will be the original bitmap.

Or, involving probably the same amount of work, do a screen capture of the bitmap (press PRINT SCREEN on your keyboard) then paste into a bitmap editing program, then crop it out from everything else.