I've read how to edit a button in Ribbon tab for a specific Excel sheet, but where are the buttons for Excel in general?

I've managed to stumble through creating the new button clicks that I want, but now I want to import the actual button image.

As I understand it, I have to edit the code for the existing button in Custom UI Editor (which I have downloaded) but I don't know which file to open.

I'm reading a helluva lot of stuff but it's over my head unfortunately.