People insert stuff from other sites like reddit.com there and 9gag put a watermark on it.
Which is what I hate about that site. They have an "original content only" rule, and people seem to think that adding a watermark makes it fit the criteria.
seems like the staff/users of kwejk.pl visit 9gag quite often
Also if you're referring to the fact that a lot of pictures are on both sites, it's most likely stolen by 9GAG, not the other way round...