If you wish to tag a member in an existing image, you can do so by going to Home > Media Manager and clicking on the relevant image.
The middle panel enables you to tag members in the image. Simply type their username and it'll make a suggestion!
The process works the exact same when uploading an image via the single uploader - you'll be presented with the option to input members usernames in the various profile type boxes, such as "model" and "photographer.
When using the batch uploader, you can tag a smaller amount of members - namely the photographer, model and MUA. However, more can be added via the process above.