Add Watermarking (with PNG 24) Ability to Current Script

I currently have a script that allows me to upload an image and re-size it. I'm am looking for someone to add watermarking to the the script without changing it's function. The script is designed to upload a users image, re-size it, and then display it for the user to download. I'm using an ajax form along side of a PHP script to currently accomplish what I'm doing. I want to add a watermark after it re-sizes the image. The script needs to be able to handle PNG-24 without changing any of the transparency of the original image or the watermark image. I also need to be able to alter the location of the watermark in the script. I'll provide you with the current script so you are able to fully test out and use. I'll expect that when it is complete it will function exactly as it currently does, but the picture presented back to the user will display the watermark on their image. By default I would like the watermark to be located in the bottom center and about 5-10px from the bottom of the image. Shouldn't be hard, from what I understand it'll need to use imagealphablending and imagecopy vs. imagecopymerge in order to support PNG-24.