Thanks.
I already have a sort of page structure with images and users. So when a user likes a image, the image id is stored in a field in the user template so i can keep track on which user likes which image. The image itself is a page containing data of the image (title, description, exif data of the image, user_id who added the image, etc).
Of course I can add the user_ids to a field to the image page itself, but that doesn't keep track if the user of which the image belongs to has 'seen' the notification.