Hey, Daughters, I found it on wsabstract.com!    We talked about depressible, clickable buttons.  There are event handlers, two new events supported by NS4.x and IE4.x: 
onMousedown and onMouseup.  (Thank Heavens IE and NS finally got together on something!) 
If you're using NS 4.x or IE4.x, this image will "depress" when you click down on it. 
Click here for example 
Love it!

As you surmised, it is done with two images, up.gif  and   down.gif. 
The "secret" is that you "pre-load" the images in the "head" section so that it appears instantaneous! 

Neat, huh?  Check this page's source code to see why clicking that first button/arrow doesn't go anywhere.  [http://wsabstract.com/howto/newtech1.shtml] 

The author asked how many times I clicked the button.  Blush.  He called it a "big, juicy button".  And how many times did you click it?  ha 

http://wsabstract.com/script/cut17.shtml
Cut & Paste "Depressible" image links
By George Chiang, host
If you want to find out how to do all sorts of neat things, 
go to the same site:
http://wsabstract.com/cutpastejava.shtml
The author also says, since it is an event handler, to "get creative".  Let's see what we can do with this.  (Hint: on a picture this size, you get a better look if you hold the button down.  In fact, if you move the cursor from over the picture before you release the click, picture #2 does not change because there is never a "mouseup".)  I can't really think why you would want something like this, but.... 
hold button down 
 
Daughter #1, get ready to blush.  Click on the picture to remove the X-rated sign! 
Be afraid.  Be very afraid! 
 
Are you ready? 
 
 
Now that was funny!
Now for the thing for which I did all this research.  CLICK INDEX BUTTON
 
Index