Knockout template using data-bind to image src property not working

I cannot see what is wrong here but the image does not display using the following Knockout template:

<script type="text/html" id="legend-template">       
    <div><input type="checkbox" data-bind="click : doSomething" ></input>
        <img width="16px" height="16px" data-bind="src: 'imagePath'" />          
        <span data-bind="text : label"> </span>
    </div>        
</script>

The object this is being bound to looks like this:

tut.myObject= function (imagePath, label) {
    this.label = ko.observable(label);
    this.imagePath = ko.observable(imagePath || liveString + '/Content/images/marker.png');   
};

tut.myObject.prototype = {
    doSomething: function () { alert("do what?");
     }
};
 

When the HTML object is rendered I see the label and clicking on the checkbox invokes doSomething.

TIA.

Only a few attributes can be bound directly; try using attr - it will let you set any attribute on an element.

<img width="16px" height="16px" data-bind="attr:{src: imagePath}" />