Also, why don't you use the "height" and "width" attributes in the <img> tag to create your thumbnail images? For example, the original image is 400x600, you could put a thumbnail using just the attributes, i.e. <img height="40" width="60" src="img.gif">.

ARRRGGGHHHH. The world would be a better place if people would STOP DOING THIS. The point of a thumbnail is to give you a preview of the image before you download the whole thing. By doing your thumbnails in this manner, you force the browser to download the entire 400x600 image, and then display it as a tiny representation of itself. You didn't save anything, you might as well have just shown it full size. It may work great for you when you view the page from your hard drive while you develop it, but it's a huge waste of time and bandwidth to do this over the net.