Sorry if this is a stupid question, but how would I make it so that when your mouse hovers over an image, the image darkens?

Here's (http://touchstone.fredfarm.com/fanart.html) a website that has this.

Thanks.

It's not stupid ^^

http://www.dynamicdrive.com/dynamicindex4/highlightgrad.htm
http://www.dynamicdrive.com/dynamicindex4/image5.htm