If your transparent background png images are been uploaded into the Nextgen gallery with a black background here is the solution: open plugins/nextgen-gallery/lib/gd.thumbnail.inc.php and scroll to the bottom of the file change the function fastimagecopyresampled to the following: thanks to oranblackwell

If you have problem with style margin: o auto, which is not working in Internet explorer. Check if you have in your header: If this is not solving your problem you should try to: remove if you have them on the first line. Set Strict XHTML/1.0 Thanks to http://www.gotsrb.info