Here are a 3 tutorials showing you how to upload, store and watermark your images using GD with PHP.

The first tutorial shows you how to upload and store images on your server using PHP & GD.
Link: Storage and re-use of images using PHP/GD – Part 1 :: gvtulder.f2o.org

The second part of the tutorial shows you how to search images, generate img tags for the image and creating pop-ups to show the larger version of the image.
Link:Storage and re-use of images using PHP/GD – Part 2 :: gvtulder.f2o.org

Now how about watermarking the images you upload? Here’s a tutorial from SitePoint showing you how to do this.
Watermark Images on the Fly in PHP [PHP & MySQL Tutorials]