For example, hover over this image:

Put this code:
<img src="YOURIMAGEURLHERE" style="filter:progid:DXImageTransform.Microsoft.BasicImage(grayscale=1)" onmouseover="this.style.filter='progid:DXImageTransform.Microsoft.BasicImage(grayscale=0)'" onmouseout="this.style.filter='progid:DXImageTransform.Microsoft.BasicImage(grayscale=1)'">