Cheese pizza is a common euphemism for child pornography online, which I believe originated from 4chan? Not sure on that part, but it's definitely a common slang for it. Now whether that's how they were using it in the emails or not, I have no idea, as I didn't look into the emails. I stopped digging into conspiracy theories around the time the whole Assange "proof-of-life" fiasco was frothing about. I've got better things I can be doing with my time.