GD Image Resizing on the fly.

Hello Everyone, I am looking for a script or guidance in where to look to figure out how to create a php script that resizes an image on the fly when it is called with some variables. To further detail, I have a shopping cart I am creating for a client -- the client has the ability to upload images, instead of creating a very complex script when he uploads the file, I figured on the shopping cart page I could grab the original file and then resize it to display a thumbnail based on 2 variables being sent W,H (Width, Height). The products are stored in a database along with the image path. I need it to display in the middle of a page so a "header( 'Content-type: image/jpeg' );" won't work for obvious reasons (header already sent). Also it will have to distinquish between Gif and Jpeg (not a problem, just preg-match the extension). I know the exact sizes that we want the shopping cart images to be. Anyone have any ideas? If you need more information just ask. Thanks everyone -- this is such a great community! Tony Devlin