Why not just use the image for the button itself?
<tdButton type="NextRound" image="C:\buttons\nextRound.gif" link="true">
The link attribute removes the button characteristics. If you want it to still be a true button (gray color, raised look, push-button effect when clicked), remove the link attribute:
<tdButton type="NextRound" image="C:\buttons\nextRound.gif">
Remember, if you use the Insert Token dialog, all of the attributes are listed with descriptions to help you out. It's a lot easier than trying to piece together the <token> stuff by hand. And don't forget, if you highlight the token code before you push the "Insert Token" button (even if you just highlight part of the token), the Insert Token dialog will open directly to the appropriate token with the attributes specified in the token code already set appropriately. Makes it very easy to edit an existing token.