How do I add photos to a gallery?

Add a folder or single photos, or drop them anywhere on the Photos tab. Folders inside become sections.

Open the gallery on its Photos tab.
Press Add a folder and choose the folder of exported photos, or Add photos to pick single files. In an empty gallery they're called Choose a folder and Choose photos.
Or drag the folder from your computer and drop it anywhere on the page when Drop to add them appears.
Folders inside the one you drop become sections named after them: Wedding/Ceremony becomes a section called Ceremony. Photos loose in the top folder go in without a section.
Follow the upload in the card in the corner. Meanwhile you can open Design or Settings: the upload carries on.

Why isn’t it working?

A photo says it couldn't be opened: The list says “the file is damaged or in a format this browser can't decode”. Export it again as a JPEG, which every browser can open, and add it again.
My phone won't let me choose a folder: Use Add photos to pick the photos instead, then make sections with New section. Whole folders are easiest from a computer.