Several users "rediscovered" a way to bypass the paint/avatar system, and once that happened it spread like wildfire--which HullBreach quickly got word of and subsequently removed both the method and the images due to their potential file sizes slowing the site down. (In addition to defeating the point of having to paint your avatar on a site named DSiPaint.) There are still some of these prohibited images floating around because Hull missed some and that's just how the avatars work.
Not going into detail about exactly how this was done, but all you need to know is that you should not.