What's the main menu background screen in Minecraft 1.8 based off?
Solution 1:
It is a mesh of 6 PNG files displayed sequentially (and with a wonderful transition!). These files are located inside your minecraft.jar inside title/bg (So yes, you can change them).
Source: I found them while browsing the jar, then stared at the background to confirm that they matched.
Also, the 2 computers I tested had the same files, so it stands to reason that minecraft.jar does not dynamically generate them.
Solution 2:
As john said it's located in the minecraft.jar file inside title/bg. Here's a little video on how to change it :D