Thanks for this, I am now just using a gif and a static png for hover state: see my fiddle http://jsfiddle.net/P4aZC/
<img id="beaker" src="http://benjaminedwardswebdesign.co.uk/testimg/BeakerStart.png" />
$(document).ready(function()
{
$("#beaker").hover(
function()
{
$(this).attr("src", "http://benjaminedwardswebdesign.co.uk/testimg/BeakerAni.gif");
},
function()
{
$(this).attr("src", "http://benjaminedwardswebdesign.co.uk/testimg/BeakerStart.png");
}
);
});