<script>
var startTime;
function imageLoaded()
{
    startTime = (new Date()).getTime();
    document.getElementById('mybutton').disabled = false;
}

function buttonClicked()
{
    var endTime = (new Date()).getTime();
    var elapsed = endTime-startTime;
    alert("elapsed time: "+elapsed);
}
</script>

<img src='https://www.google.com/images/srpr/logo11w.png' onLoad='imageLoaded()'>

<input id='mybutton' type='button' value='click me' onClick='buttonClicked()' disabled>