Adds download functionality to prettyPhoto WordPress plugin. Put the code into yours theme functions.php or the file into mu-plugins folder

prettyPhotoDownload.php

add_action('wp_footer','ls_prettyPhoto_Download');
/**
 * Adds download functionality to prettyPhoto plugin
 * @author lenasterg
 */
function ls_prettyPhoto_Download() {
echo '
<style>.pp_download{ background: #ccc;  border-radius: 3px;   height: 25px;   padding: 4px;}</style>

<script>
jQuery(document).ready(function() {
   jQuery("a[rel^=\'prettyPhoto\']").prettyPhoto({
   changepicturecallback: function(){
       jQuery(".pp_social").append("<span class=\'pp_download_place\'></span>");
        $getit= settings.image_markup.replace(/{path}/g,pp_images[set_position]);
      jQuery(".pp_pic_holder").find(\'.pp_details .pp_download_place\').html(\'<a class="pp_download" href="\'+pp_images[set_position]+\'" download>Download</a>\');
        }
    });
});
</script>';
}