I'm not absolutely sure this will work, but try this:
Go to data/textures/TNormal/tex
Delete the following files:
druck_radio_7
druck_radio_9
druck_radio_21
IIRC, the default is a poster of Dönitz.
If that doesn't work, look for the "Dönitz Fly" mod or some such.
Ultimately, those files are where the poster is located for any mod. The originals are located in data/textures/TLowRes/tex.
Cheers.
Update:
Go
here.
Download one (or all) of the propaganda files. Use what you want from there. They are easily portable to the druck_radio_9 and druck_radio_21 files.