A nice, large theatrical Wing Commander III poster was given away with pre-orders of WC3 at certain stores, back in 1994. They show up on eBay once in a blue moon, and are very nice.
Wing Commander IV (the 'outer' red box art) posters were given away by Origin in contests and (probably) at trade shows and such. They were also given to some stores to promote the game. Pretty tough to find -- I'm still looking, myself.
I have absolutely no idea how the Prophecy poster was distributed... I just know that Captain Johnny has one. *Probably* just given away at trade shows (like the mousepads and Prophecy water and what-not).
Wing Commander movie posters come in all shapes and sizes. There's two 'styles', though: the awesome French yellow Rapier poster and the painfully non-awesome American giant Freddie Prinze Jr. ear poster.
What else... hmm...
An issue of 'Sci Fi Teen' came with a Wing Commander Movie pinup... which turned out to be a kind of awesome poster of Freddie-Prinze-Jr in Space Armor.
An issue of CGW came with an awesome Prophecy 'Midway' poster -- the same image that's in the Prophecy box's "flap".
The WC Movie brought us a pair of posters of movie-Blair and movie-Maniac. Which is great, if you're a teen girl or a creepy gay guy.
The Privateer 2 official guide came with a poster of all the ships.
Wing Commander 3 Premiere and SAMs Club editions came with folded WC3 artwork posters.
Privateer 2: Deluxe came with a Privateer 2 poster (American box artwork).
When WCIV came out, a company sold 'ChromArt'... which was... chrome-inlayed artwork from the games. Four pieces were available, two from WC3 and two from WC4. (the box art for each game, a picture of a Dragon on Telamon and the Excalibur leaving Kilrah).
Rare-er stuff: Bill Forstchen has (used to have, at least) an 'End Run' cover poster in his office. Maybe a trade show thing?
Chris Roberts has a 'Wingleader' (WC1 art, different name) poster... not sure if these were produced in any quantities, or if a regular 'Wing Commander' poster was ever made (Origin sold posters direct back then, but they never did a WC1).