def grab_file(source_file)
      source_name = File.join(PictureTag.config.source_dir, source_file)

      unless File.exist? source_name
        raise "Jekyll Picture Tag could not find #{source_name}."
      end

      source_name
    end
def grab_file(source_file)
      source_name = File.join(PictureTag.config.source_dir, source_file)

      unless File.exist? source_name
        raise "Jekyll Picture Tag could not find #{source_name}."
      end

      source_name
    end
Turn a relative filename into an absolute one, and make sure it exists.