The scene where the wampas break in was partly filmed, but I don't believe Luke was involved. The way it went was that R2 is rolling down a hallway and then notices something behind him--there is a shot of this in one of the trailers. What he sees is a wampa, and then rebel troops start firing at it. I guess the new deleted scene trailer shot shows R2 running away while this is happening. Anyway, I'm not positive, but I thought the gun cannon was used by rebel troopers in this scene. That would explain why the prop was built in the first place.

The shot of Luke with it is just a publicity photo, and publicity photos don't have anything to do with actual scenes, as long as they look cool. The gun cannon was a good prop to use, so they used it. Luke is in his flight uniform, which he only wears in Echo base when he is heading to his snowspeeder. That whole section of the film is supposed to be kind of quiet and sweet, with Luke saying goodbye to Han. Having a random Wampa attack scene (yet ANOTHER one) where Luke randomly mans some gun cannon while on his way to say goodbye doesn't make any narrative or editorial sense. The only evidence that ever tried to link that photo to a filmed scene is a recollection by a guy who wasn't there in 1979 who is basing this on a photo CD that Lucasfilm gave him. Probably he saw that publicity photo, and then other photos of the wampa being shot, and decided they were the same scene.