yfrog doesn't seem to have it documented how to get an image URL given a yfrog URL, such as you might get from twitter's API.
this gem is intended to abstract this process for robustness.
$ gem install yfrog
require 'yfrog' valid_url = YFrog.valid_url?("http://yfrog.com/kjugqgbj") image_url = YFrog.image_url_for("http://yfrog.com/kjugqgbj")