A POTD is an acronym for "Photo of the Day", "Picture of the Day, or "Poll of the Day". It is a term most commonly used by online digital photography communities as an honor to outstanding photos that are selected daily from among that community's membership. They can either be tagged as POTD or displayed in a special page on that site.
In addition to digital photography sites, other focused topic sites may solicit and display chosen POTDs that relate to their given topic. Sites pertaining to news, weather, animals, nature, videogames and sports are among the many that display such POTD selections. In the case of videogames, a POTD refers to a screenshot from a (usually) upcoming release.
Additionally, the GameFAQs social board for their Poll of the Day is frequently abbreviated to PotD, with special consideration to the capitalisation of the letters.