How can I add extra text fields to the image field when adding/ editing nodes? What is the best way to add additional text fields to the image module, other than the title and alt value, when adding/ editing content? Is there a module which already does this?

Edit

I've read through Add custom fields to image field and unfortunately this fella got stuck and I guess this was at 7 times. The fieldable files module is no big help. It does a lot of magic but it doesnt effect images added via node entity form.
Image is a file entity and to make it fieldable you can use this module:

File Entity (fieldable files)

From the documentation:


  File Entity
  
  The File entity module extends the capabilities of the core File
  module.
  
  File Entity defines 5 File Types - Application, Audio, Image, Text,
  Video. The Media module adds an additional File Type - Other
  (currently these File Types are hard coded and cannot be changed).
  
  When files are uploaded they are automatically assigned the
  appropriate File Type based on the file suffix.
  
  The File Types as currently defined are:
  
  Images - .png, .gif, .jpg, .jpeg, .ico Videos - .mov, .mpg, .flv,
  .m4v, .mp4, ,ogg, .ovg, .wmv Sound files - .mp3 Text - .txt Document
  files - .doc, .xls, .pdf, .ppt, .pps, .odt, .ods, .odp Other - ??
  
  Each File Type can have fields. This allows for additional information to
  be stored along with a media file such as Title, Description, Taxonomy
  References, Node References, Location, Date created, License
  information etc.