Images : Upload, Create Thumbnail, Manipulate
 

I want to write a function to allow the user to upload images, create a number of different thumbnails of various sizes, and to manipulate the thumbnails (putting rounded corners on them, watermarks, etc).

Anyone know of any good tutorials, etc?

You would most likely be wanting to look at the PHP GD Functions. They are the functions relating to image manipulation within PHP (unless if you use ImageMagick of course).