The photo widget on photo.se seems to only exist on that site. My site (Arqade) has somewhat recently started doing Screenshot of the Week contests, and the widget would potentially be a good fit for promoting it on our site. We also have quite a lot of support from the community for adding it.
However, there doesn't seem to be any documentation on how the widget works. The best I can find is this post from '11, saying that the aspect ratio changed.
What features does the photo widget have, and how does it work?