Breakpoints Drupal 7 – add to existing fields

I’m trying to set up responsive images on my site.  I’ve just added the Breakpoints module. Drupal 7. 

Need to add Image styles via the cog on the right hand side on manage display with editing a content type.  The cog shows for new fields but I can’t see how to add to existing fields.

Any idea?  

Drupal version: