Adding Content Groups to Image Layers

Go into the template as an administrator on the back end.
Click on an existing image layer or create a new one.
Select “Content Groups” from the Content Source drop-down menu:
Click into the "Select Content Group" field:
The available content groups will appear in a dropdown menu:
Scroll through the list or type in the name of the group (the field with the cursor is a search field).
When the correct group is highlighted click the group to apply it to the image layer:
This content group is applied to the image layer for both admins and end users:

How to add a content group to an image layer.