If I supply my own image for the TimePopupButton like so:
<TimePopupButton ImageUrl="~/Images/TimePopup16.png">
The image displays fine, but on hovering over the button, no image is displayed. I have to add the HoverImageUrl attribute as well. If not specified, the HoverImageUrl should use the same image.
Additionally, the image tag inserted when I supply my own image does not have the border="0" attributes or any border css applied, so it displays (in IE at least) the heavy blue border that images within anchor have by default. I had to add:
$( '.RadPicker .rcTable a>img' ).css( { 'border': '0px none transparent' } );
to my page initialization code to get rid of it.