The recreate option for images currently does not recreate the detailed image.( atleast in my installation 
)
It would be nice if Joomgallery would check if there is an original image and if so recreate the thumb AND detailed, or ask if you which you want to rebuild.
nevermind need coffee, I uploaded a new image on the first field on 'Replace image files' that i saw which was the thumbnail instead of original. 
recreate just works fine. 