According to this article the image of Mario is trademarked. Since you are running a gaming website it sounds like this could lead to confusion about whether your website is endorsed by or part of Nintendo, and hence be a trademark infringement. However that would depend on the details of what your website does. If you provide games then its more likely to be an infringement than if you merely write about them. It might also help if you stuck a disclaimer somewhere prominent saying that your graphics are not used to imply endorsement.
There is something called "nominative fair use" for trademarks, which means that merely referring to something by its trademark is not an infringement. You could probably argue that your use of Mario along with lots of other video game characters is merely referring to Nintendo games in a visual medium.
From a copyright point of view, the image of Mario is copyright owned by (presumably) Nintendo. However you might well be able to argue fair use; it all depends on the exact circumstances.
From a practical point of view, sticking up a sign to Nintendo's attack lawyers saying "sue me" is probably not a wise move, regardless of the merits of your case.