Images

Preloading images

hiddenImg = new Image();               // Create a new image object.
hiddenImg.src = "img.gif";             // Download the image.
document.myImage.src = hiddenImg.src;  // Display the image.

Mouse rollover

<a href="somewhere.html" 
  onMouseOver="document.myImage.src='imgOn.gif'; return true;" 
  onMouseOut="document.myImage.src='imgOff.gif'; return true;">
<img src="imgOff.gif" name="myImage" width=100 height=6 border="0"></a>