From the site:

...Historypin is made up of photographic images, videos, audio clips and descriptive and narrative text.

Photographic images can be pinned directly to the Historypin map by users. These images can be of any location - outdoors or indoors - at any time in the past.

Some of these images, if they are taken outdoors, at street level and at certain angles, will be able to be layered onto Street View (this is a bonus, not a requirement).

Audio and video content can be pinned to the map by users. These should be pinned to the location and date where they were recorded...

HistoryPin's mobile app reveals past photos taken near your current location and allows you to view them layered over the modern scene.