FlexColumn stack
The FlexColumn stack makes it easy to create 'unlimited' columns. Just drop floating stacks in a FlexColumn stack - size them - and you've got a very flexible column system.
The FlexColumn has some advanced background image settings allowing you to place images in each corner. Or anywhere else.
If you place two FlexColumns on top of each other - you can mix backgrounds. This is what I have done in this example.