In the search box allow to search by the custom fields you added to the photos.
This way the custom fields would work like tags and you would be able to filter by then.
It would be a great alternative to filtering by image name, because the image names of the uploaded are usually just not meaningful (just the name that the camera gave to the file).
In the simplest implementation, it could be adding support in the search box for something like: "tag:value" and these would search by the exact text given in "value" only inside the custom fields defined.
A more advanced implementation could allow to give each custom field a shortname and then you could search in a more advanced way using something like: "tag:shortname:value" and it will return just the photos that have this specific tag.