[Plugin: NextGEN Gallery] when thumbnails are not showing…

… you should fix a bug in nextgen gallery:

in file wp-content/plugins/nextgen-gallery/lib/gd.thumbnail.inc.php in line 603:

original:
ImageJpeg($this->newImage,'',$quality);

please change this to:
ImageJpeg($this->newImage,null,$quality);

Due to php documentation it’s the more correct way.

http://wordpress.org/extend/plugins/nextgen-gallery/