If this is all new for you read the information on this page first : Change the Ribbon in Mac Excel 2016, you will also find a few example files on that page.
Note: You see that I use imageMso in the xml to point to a built-in icon in the examples. There is no list of built-in icons for the Mac on this moment on the Microsoft site. But I try to make a list myself of every built-in icon that is working in Mac Office 2016. Below you can download an Add-in that show you the 1671 icons that you can use in Mac Office 2016. Note: The original Add-in was created for Excel for Windows by Jim Rech, I make some changes to it so we can use it in Mac Excel 2016 now. In the Win Office 2010 version Jim found 3459 icons but most of them not show up in the Mac Office Ribbon, maybe Microsoft will add more icons in the future. Note: when you open the add-in there is a new tab on the Ribbon named "ImageMso's Mac Office 2016" and you find the 1671 icons in the Ribbon tab.
When you point to a icon it show you the imageMso name, click on it to copy it to the clipboard so you can past the name where you want so you can use it in your RibbonX. When you hold the Shift key when you press the Previous and Next Group buttons it jump to the begin or end of the index.
Note: You install the add-in like any other add-in with Tools>Excel Add-ins
Note : You can also use some built-in control idMso's as imageMso names, so if you want to use a built-in button icon in your custom ribbon you can search for that idMso name and use that as imageMso in your RibbonX.
Note: If you are a Windows user there is also a Windows version of the Add-in on my site