if (document.images) { 
  ImageName0 = new Image(); 
  ImageName0.src = "textup.jpg"; 
  ImageName1 = new Image(); 
  ImageName1.src = "textdown.jpg"; 

  AnotherImage0 = new Image(); 
  AnotherImage0.src = "ojectsup.jpg"; 
  AnotherImage1 = new Image(); 
  AnotherImage1.src = "objectsdown.jpg"; 
} 

function change(Name,Image,No) { 
if (!document.images) {} 
else document [Name].src = eval(Image + No + ".src"); 
}

<a href="page1.htm" 
  onMouseOver="change('ImageName_img','ImageName',1)" 
  onMouseOut="change('ImageName_img','ImageName',0)">
  <img src="textup.jpg" name="ImageName_img" border="0">
</a>

<a href="page2.htm" 
  onMouseOver="change('otherImage1._img','AnotherImage',1)" 
  onMouseOut="change('otherImage1._img','AnotherImage',0)">
  <img src="ojectsup.jpg" otherImage1._img" border="0">
</a>