in the gallery/galleries/slideshow_wide/index.php file i change the following line:
echo(pg_getCachedResizedImageHTML(ob_get_clean(), "" . realpath( $galleries_dir ."../images/" . $row_gallery['folder_name'] . "/cache"), "". $fullsizeInfo['resize'] ."", "",true, false, $rotateAngle, $watermarkOpts));
to:
echo(pg_getCachedResizedImageHTML(ob_get_clean(), "" . realpath( $galleries_dir ."../images/" . $row_gallery['folder_name'] . "/cache"), "". $fullsizeInfo['resize'] ."", "#FFFFFF",true, false, $rotateAngle, $watermarkOpts));
Then in the images folder, locate your gallery and delete the cache folder to have the images regenerated.