Find the folder where MODO is installed, and look for the following folder:how do you upload a custom mat like that?
%WHEREVERTHEUCKYOUINSTALLEDTHECLIENT%\WotC\MTGO\Graphics\UI\backgrounds\gamemat
Save the image there, making sure that you add _a to the file name right before the dot. Run the client and you can now pick your image just like any other of the pre-installed ones.