How to add images to Fullwidth Gallery?

This seems like a simple question, but none of the Highend theme documentation or YouTube videos actually tell you HOW to add images to a Fullwidth Gallery section. Searching on the user forums also yields nothing. It seems like an assumption that everyone should automatically know how, but adding images to the Content section doesn’t work, and I don’t see any other way to add content to a gallery. This is a serious shortcoming in the documentation.