Add a cover image
Give a collection the picture that stands for it wherever it is offered, and replace or remove it later.
A collection's cover is the picture that stands for it in a grid of collections, so it is doing the same job as a shop sign. One image per collection, set from the collection itself.

Upload a cover
Go to Collections and open the collection.
Find Cover image and select Upload.
Choose the file. It appears in place once it has uploaded.
Replace or remove one
Select Replace to swap the picture for another. The delete control removes it, and Storefront asks you to confirm first, because a collection without a cover falls back to a plain card wherever it is shown.