Apparently MINT, Cinnamon doesn't support having two backgrounds, wallpapers with two or more monitors, WHY? Shame really, as far as I can see its the only thing that lets Cinnamon down.
I use DisplayFusion with my Windows 8 machine and I gladly pay for this software - this software supports a number of monitors with different wallpapers on each. I have contacted DisplayFusion and there doesn't appear to be a commintment for Linux.
My MACs support different wallpapers off the shelf. So does KDE which I used to use, but I much prefer Cinnamon. I know I could revert back to KDE, but why should I leave a better environment.
In some respects I understand the developers of Cinnamon not taking on the task, perhaps they're happy with one monitor or the same image on both.