I've always thought it was like "Utopia" but for photos.
I always thought that too, and the official capitalization appears to be Potopea everywhere. But it has a performance test tool called PeaMark, which has me questioning my assumption.
I thought it was "Pea" in the sense that the site is a small .js codebase. Now I'm very curious of the lore...are all 3 of us wrong?
Ivan answered this in one of his AMAs on Reddit[1] a couple of years ago. He used "Pea" because "Apple" was already taken.