Try these: http://www.gallerycms.com/ http://www.zenphoto.org/ http://microstockinsider.com/guides/photo-cms-content-management-systems https://www.google.com/search?q=Gallery+CMS
My suggestion will be to go for KREATIO. Its some of the features are especialy built for image management. Creating image of various size is possible and is very easy to manage. One major photography site is using it.