How to Make File Description Field Required?
depending on your needs, this could be accomplished using Field Collection, adding an image field and a description field (which can be made required) to the collection:


  Provides a field-collection field, to which any number of fields can
  be attached.
  
  A field collection is internally represented as an entity, which is
  embedded in the host entity. Thus, if desired field collections may be
  viewed and edited separately too.