Yeah, with even an average inkjet printer and some basic graphics programs, it's not hard to print something scaled down to
40K scale.
The main issue is finding suitable posters, or graphics that could be turned into posters. Hmm, Games Workshop themselves have had a few (often appearing as a page in White Dwarf) - although how available they now are is a different story...
A quick Google image search on '
40K poster' gives some (amongst all the dross), e.g.: