AFAIK, it can't be done in HTML/CSS. Your options are basically javascript or flash. The javascript way would be to install a javascript framework like
jQuery or
Scriptaculous or
Mootools (and oy! don't ask which is best, what a can of worms that is! I use scriptaculous because that is integrated with my framework.)
The JS framework has functions which let you add "effects" to DOM elements (like a an IMG or a DIV containing an image). These effects are achieved by re-styling the CSS of the element many times very quickly, so it looks like a smooth transition, or fade, or color shift, or what have you.
For a picture switcher, you'll have to figure out a way to get all the pictures in the same spot as well. If you don't have a lot of pics, you could load them all into an array. If you do have a large pic library and don't want to the user to have to download them all, you're looking at AJAX requests.
Alternatively, you might be able to find a standalone javascript that does what you're looking for, probably as part of a larger javascript app that does what's lightbox effects, which is a fancy way of saying a js that displays pictures prettily.
What you're trying to do sounds simple, but it really isn't that simple. So if you're still learning html/css, you should probably put it aside for later.
Unless some clever chap around here knows a better way...
edit: Angelwatt beat me to it