I have dual monitors with Windows 8.1 and it sounds like yours is completely different to mine.. Is your computer defaulting to the "Metro" interface or the Windows desktop? Mine defaults to the desktop, which is what I prefer. This might be the result of Windows updates, I'm not sure. I've never had it defaulting to Metro (this PC has been setup with Windows 8.1 from the outset, not upgraded).
If you want to change the background on the Metro theme, on my computer you press WinKey and Q to open the "Search Charm", and search for "Personalise" and the first option displayed is "Change the background and colours on Start". There is also the same for the Lock Screen which is also known as the log in page.
This allows you to change the Metro theme like so..
If you are in Desktop mode, to change the background just right click anywhere on the desktop and choose "Personalise". Or "Personalize" in the US
Edited by jonuk76, 15 August 2014 - 09:14 AM.