You could either keep a record of which images have been viewed in a session (meaning it would only count once per opening/closing of browser),
Or in a cookie, which would count only one computer once every.. depending on how you set it up, when to expire etc.
I'd personally do the session one, meaning you could set up an array to keep in a $_SESSION variable.. call it viewed, or something.. and just add the ID of the image to it when it's viewed.. and beforehand say, increase the view count only if it doesn't exist in the array variable