It's the "signon" screen. It's directly eating it's way through the exe and is stored within the file. If you check signon.c or signon.cpp (wolf4sdl) in the source it will be the actual picture you see there. "Press a Key" and "working..." at the bottom are text strings in the code. You can find them in WL_MAIN.C.. under the "FinishSignon" function.
One of the two converters (the bmp2c is older and was used for most DOS mods) can convert an image to the signon source file. You will have to compile the code for the new picture of course.
http://www.belowe.com/bmp2c.rar (supports only wolf3d palette)
If you want to have more fun you can always take the palette values and convert it manually by hand in a text editor : P