How to install a javascript widget for buythisimage?

buythisimage.com is a new service offering a little widget to allow ordering prints from images on blogs & web sites.

It seems to require inserting this js code into the template which displays an image:

<script type="text/javascript" src="http://username.buythisimage.com/BuyPhoto/js?service=generic"></script>

Can anyone tell me which template file (or files) in G2 need to have this added, and where in those files this code should go?

TIA!