def url(name = nil)
      if name
        if version_identifiers.include?(name.to_s)
          self[:@versions][name.to_s][:@url]
        else
          raise "Invalid Image Version identifier: '#{name}' (must be #{version_identifiers.join(', ')})"
        end
      else
        self[:@url]
      end
    end
def url(name = nil)
      if name
        if version_identifiers.include?(name.to_s)
          self[:@versions][name.to_s][:@url]
        else
          raise "Invalid Image Version identifier: '#{name}' (must be #{version_identifiers.join(', ')})"
        end
      else
        self[:@url]
      end
    end
Returns the URL of an Image or the URL of a version if a string or symbol is passed