First, set up your collections.
See this FAQ:
How to Group your Artwork into Collections
To assign all images to their collection:
1. Only those images NOT already assigned to a collection will be displayed.
2. You CANNOT assign one image to more than one collection.
This also works, but is a bit more cumbersome:
Once you set up and use collections, we recommend that all art be assigned to a collection. Any artwork not assigned to a collection will not display on your website.
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