Yes, the Windows 7 beta wallpaper was a picture of a betta fish
It wasn’t long before people realized that the fish in the default wallpaper for the Windows 7 beta was a betta fish. This was intended to be a cute little pun, though some people took the semiotics to an extreme, Dude, this is Windows, not The Da Vinci Code. It’s not like the people who chose the wallpaper are using a backchannel to pass secret messages to you like “I know I’m not supposed to tell you, but here’s the Windows product schedule” or “Help, I’m trapped in a wallpaper factory!” They’re just having a bit of fun. I have yet to see anybody point out that the fish was blowing seven bubbles. And no, I don’t know what it “signifies”.
(I always wonder about the people who claim that the government embeds subtle messages about their complex conspiracies in plain sight. If you want to keep a secret, you don’t run around talking about it!)