It's squirrelbacca fighting the stormtroopers. I'm guessing it's the incongruous juxtaposition of images that makes it humorous. Personally I don't think it's that funny either. And those rugers are very inaccurate.
Imagine that over your fireplace, friends are over, and you say "THIS is my prize possession. Is it not beautiful? What do you think?" and seeing the looks on their faces as you try to keep from cracking a smile.