I don't believe that's an easy thing to do with what you have.
Using a SwapImage and the href being "#", the problem is that if you create a CSS rule for a visited image link to fade the image (I don't even know if that's possible), since they all have the parent page as the link ref (#) once you hit one of them ALL of them would fade.
There may be a way to do this, but with CSS and HTML alone, it will be difficult to impossible.
This is done with the "Opacity" property. See demos with code examples below:
There is a commercial Behavior called Transparency at http://www.actionscafe.com/ that can do this for you. You would have to attach 5 onClick Transparency Behaviors to each thumbnail in your strip. The first would dim the clicked image, the other 4 would restore the remaining thumbnails to 100% opacity. You need all 5 because you don't know in what order the thumbnails were clicked.
Another solution is called FileTabs. This Behavior treats images like 2 or 3 state rollover radio buttons. The clicked thumbnail stays in the second or third state - dimmed in your case - and the previous thumbnail returns to the first state. You would have to create a 2nd, dimmed image for each thumbnail of course.
You might want to have a look at these if you can't find some free script.
I went to http://www.w3schools.com/Css/css_image_transparency.asp and tried out that transparency code. It works great except that when the page loads initally all of the thumbnails are dark (transparent). What am I doing wrong? Please help. Here's the link to my page:
I added this code:
and it seemed to fix my problem. Thanks for all of everyone's help!
Can't seem to delete this message.
I looked at your finished page and the thumbnails don't seem to work the way you described it in your original post. I thought you were looking for a radio rollover effect like this... http://www.actionscafe.com/csruskin/
Hope you don't mind my using your material in this demo.
This page doesn't use fading. It uses the FileTabs Behavior and rollover thumbnail images created in Photoshop. I'm only posting this reply FYI, in case you need the radio rollover effect on a future job.
I actually did want them to darken and by fading the thumbnail image they would since I have a black background. Thanks for your demo - I will probably use it on some other website in the near distant future!