Basically it says, don't throw your garbage in the river.  But what is funny about it is the guy throwing the garbage is a cartoon version of a typical yakuza (Japanese mafia) guy.  Noone wears sunglasses except the mob.  Anyways it is a nice way of telling people to quit polluting the river while at the same time ignoring that there is actually a problem by blaming it on the yakuza who everyone knows are bad and do criminal acts anyways.  Maybe you have to have lived here to get it.