To do Alt + P G on Mac in Excel, download and install Accelerator Keys, then press Option, followed by P and G.
Command: Background
This is how to do Alt + P G on Mac in Excel with Accelerator Keys, the native ribbon and alt key shortcut solution for Mac users who get more done with Excel.
Alt + P G opens a dialog to select an image as the worksheet background. The background image tiles behind your cells for visual effect on screen, though it does not print. Use Delete Background to remove it.