You can assign one or more tags to images while uploading by clicking on More upload options... in the Upload Images panel and fill the Tags input field before uploading an image.
In addition you can add, modify and remove tags for already uploaded images, by using our API. Here's our PHP API documentation regarding tags management:
It's on our road-map to support managing tags interactively through the Dashboard.
You can now manage your tags interactively through your Dashboard 'Media library'.
Simply select your image, and click Add tag to specify the desired tag for it.
Repeat this to add more tags:
Click on the 'x' to remove a tag: