Let's take a look at how you apply an Index to an Image or Folder.
You'll first need to navigate within Imaging to the Folder or Image you'd like to index. Indexes can be applied from the image or folder's Details Panel. To access that panel, you can click anywhere on the row for that object to the right of the object's name:
Once you've accessed that panel, you'll see an Indexes area at the bottom. If you click the "+" icon there, you'll see an Apply Indexes area open up, allowing you select your desired Index from those available in your Imaging account.
If you were to add four Indexes for this image, it might look like the below after you have saved your changes and returned to this area:
Note as well that if you hover over any of these applied indexes you'll see a "-" at the end their row - clicking that icon will archive the index for that client, removing it from their active indexes.