The criteria by which a photo is picked are likely to change over time, and there may well be some subscriber controls over how photos are picked added soon.
Having said that, at the moment:
- likes are the most important factor in choosing a photo,
- followed by number of comments.
this means that the Pastcard you get for a month will be one of your most liked photos in that month. If there aren't any with many likes, it'll be one with a relatively high number of comments.