You don't use seafood, it's just pictures. Surely the concept of symbols and logos isn't utterly foreign to you, I know wizards use them as well.

And you've clearly learned nothing from your class if you honestly think that's all Muggles do. Or if you think they don't believe in magic - lots of them do, actually. It doesn't give them magical blood, though. And you seem to have forgotten that it's wizards who have hidden themselves from Muggles, not Muggles who have stopped believing.