Please review the ideas at mw:Extension:FileAnnotations/Design and give feedback on the talkpage.
FileAnnotations is an extension to allow annotations of files in a structured way.
To use the extension, go to any file page. The file may have annotations on it already - if so, you can hover over them to see their content.
Several kinds of annotations have special rendering modes:
You can click on the "Add file annotation" button below the image to create a new annotation on the file page. You can also edit any existing annotations on the file page by clicking "edit". This makes the annotation draggable and resizable, and gives you a text box with "Save" and "Cancel" buttons.
It's also possible to edit the annotations in their JSON format. They are stored in the File annotations namespace, with the same page name as the file.