First you will need to enable Extra in the Visual Effects settings. To do this you will need to right click on the Desktop > Change Desktop Background > Visual Effects tab. Then just click the circle that says Extra. Now just click close.
Extra Effects Enabled
You will now need to install the Compiz configuration settings manager. To do this you will need to open up the Synaptic Package Manager by navigating to System > Administration > Synaptic Package Manager. Now type Compiz in the Quick Search bar. Then mark compizconfig-settings-manager for installation. After that just hit apply and it will now download and install the package.
Synaptic Package Manager
Now you will need to navigate to System > Preferences > Compizconfig Settings Manager. Now you will need to enable the Desktop Cube by putting a check in the little box by Desktop Cube. Then a little box will come up asking if you want to Disable the Desktop Wall and you click Disable Desktop Wall. While you are still in Compizconfig Settings Manager you will also want to put a check in Rotate Cube and Cube Reflection and Deformation. Now to add caps to the cube you will need to double click on Cube Reflection and Deformation.
Cube Reflection and Deformation Settings
Now you will need to click on Appearance and it will then drop down with some settings that you can change. To change the default caps you will need to click on New for the top and bottom caps and all you need to do is browse to a image that you would like to set as the top and bottom cap for your cube.
Appearance Settings Drop Down
Now if you would like to have something other than just the cube you can navigate to the Deformation tab and then from there you can select None, Cylinder, or Sphere from the drop down menu near the top of the window.
Now you are done setting up your Cube, Cylinder, or Sphere. There are many other settings that you can play around with but the ones I have shown here are just to get you started. Now to view your new effect you can either hold down Ctrl + Alt then click on the desktop or you can click on the desktop using the middle scroll button on your mouse if you have one. So now go have fun with you new settings and customize it to how you like it.