I am looking for some software that will be able to facilitate the
following functionality.

user registration 
uploading of pictures, 
allow users to give pictures ratings, 
automatically collate the most viewed and highest rating pictures on a
"cover page" with a smaller version of the picture,
allow viewers to click on a selected picture on the cover page for a
larger view and then allow them to make comments/view previous
comments by other user regarding the selected picture.
search functionality based on pre-determined variables or user input key words.

I think You are telling about Copermine Gallery. It includes all the
features which you post in the question.
See the Demo here http://www.page3.in/gallery/  and You can get it
from http://coppermine.sourceforge.net/ with lots of Skins.