I have a website where users may upload images...

I need to add my logo (watermark) to the images once they are uploaded.

How can I do so?

And it is important that the watermark is in a corner where it will be visible, for example I have seen websites which generates a watermark on the fly, and puts the mark wherever the background of the main image is "the same color" so the watermark sticks out if you know what I mean.

Anybody have a good tutorial or article about this? Or know of any function in php which I would need to find the position of the watermark?