Png Transparency Resize problem

Hello. When a photo which is png and has some transparent areas is uploaded to the site, the system creates the thumb but with black background on those transparent areas. Why is that and how can I fix it?

Thanks.

Don't upload .png pictures to the site.  Duh!

Thanks but I need a solution not an advice. It is social site and I cant predict which user will uload which type of image. Anyway I want to support png transparency.

You can control it by not allowing it.  Duh!  I see your site will last long... Might as well quit now...

An easy solution is to replace the black background with the background color of your site...

(It's quite easy to choose which color will replace transparent area)

See http://www.php.net/gd