Provide the ability to “tag” or “label” photos with multiple arbitrary terms.
There should be a new management page in the admin section, and appropriate permissions. The management page should allow creation of new labels, and removal/rename of existing labels.
Underneath the photo metadata fields (album photos tab) provide an auto-complete suggestion box where you can type labels in - comma separated? New labels get added automatically, existing labels get linked.
In the gallery, labels should be listed below the photo (or the metadata?) with links to the relevant “label” page - which follows the same design as the “show album” gallery page.