If you take a look at flickr.rb, you'll find its photos method simply collects the photo ids returned by the Flickr method flickr.photos.search.
That Flickr method actually provides you with the total number of matched items, but this information is not used by flickr.rb. You may want to modify the photos method a bit so that it returns that number (and perhaps other pagination info too, along with the photos). Then just as Dogulas Shearer has said, you can go back and forth using the page parameter.
d.