You should probably consult the documentation for the drupal VotingAPI
http://groups.drupal.org/taxonomy/term/233
You can take Nikit's approach of rethinking what is really an image gallery. I am assuming you problem with separate nodes is not the quantity of nodes created, but the tedious process to upload a single image one by one.
Check out a module like Image Fupload which can handle the uploading of multiple images and the creation of nodes for each of those images. Plus if you use it through the image module it can handle tagging all those images to same taxonomy (which could be your gallery-name, instead of using a nodereference in Nikit example)
another last option, try to find some random javascript that can do the rating independent of drupal, yes you will not be able sort in Views by most voted, but if you simply just want a 5 star rating and nothing else thats an option. I am sure some googling will find a sufficient script to handle that on the side.