Why add descriptions? 
When you add descriptions to photographs, you are not doing it yourself, but rather for people in the present or future who would not know what they are looking at or are looking for a photograph of something using a search tool such as Google. In other words, although photo descriptions are short, in relation to other writing assignments such as paragraphs and essays, they require the same amount or more care and consideration. You must guess what someone ELSE needs to read, rather than what you want to say. Descriptions of photos in archives require more care than newspaper captions, as there is no other text (such as an article) to use as context. Archive descriptions must give the complete information by itself.
- Always identify the main people in the photograph. Of the person/people are notable, provide names and a couple of words (novelist, first winner of X prize, president of SALALM). If more than one named person, indicate where each person is (John Wolf (left), Eric Erickson (speaking at podium)) If not, provide a specific category (medical students at ITESM)
- What is/are the person/people doing? Use present tense. Add what happened just before or just after the photograph was taken, if appropriate (e.g. people, checking damage after tornado destroyed home). Add reason for the action when appropriate (people in Cuautitlan in procession with an image of the Virgin of Sorrows for Good Friday)
- Photos of objects also need identification, by name if notable itself (e.g. the Tizoc Stone) or as a member of a category (e.g. a Nikon D60 from 2008 or amate paper made by residents of San Pablito, Puebla). If the object is artistic in nature (and it is legal to reproduce the image) an author's name is necessary (e.g. Mona Lisa by Leonardo Da Vinci). Even for non artistic creations, (e.g. a school science project) adding the creator's name is always in good form.
- Where is the scene? This is important for just about any non-astronomical photo, especially if taken outdoors. Be as specific as possible (a photojournalism class at UNAM Mexico City, Senator Mary Landieu at the Netherlands Citizen Protection Conference 2012 in Amsterdam) Commons also allows the addition of geocoordinates to photos if your camera does not do so automatically.
- Commons is not looking for poetry or explanation why the photo is important or great. The photo itself will do that. Avoid phrases like wonderful vivid view of…, the peacock’s tail is like a rainbow, Tom Jones feels sad about…
- Descriptions are written for both the person looking at the photograph and for those looking for photographs. Your description should contain as many relevant keywords as possible (names, locations, actions, etc) so that they can be found in Google or Bing searches. For someone looking at the photograph, assume they are a foreigner without your local knowledge of the subject. Good descriptions also help with categorization and re-categorization efforts later.
- Commons requires a description which can be in any of the 270+ languages that Wikipedia supports. However, if you can also add an English description your photo will be easier for others to use and much more likely to appear in their searches.
English: Dr. Ricardo Ramirez Mendoza director of ITESM-Campus Ciudad de Mexico gives a status report to professionals of the campus
English: American football game between the ITESM-Campus Ciudad de Mexico youth team división C and that of ITESM Campus Toluca at the latter's home field
English: Photographer with telephoto lens taking pictures of an American football game between the ITESM-Campus Ciudad de Mexico youth team division A and that Gamos UMA
(what could be added to improve the description? Answer:Name and maker of lens? Thought question, why not the name and maker of the camera?)
English: A patient receiving blood after a high-risk surgery
(what could be done to improve the description? (Answers: where IV is inserted, what is being given through the IV, where treatment is taking place) (Thought question: Is "high-risk surgery" needed in this case? Why or why not?)
Español: La procesión del domingo de ramos comienza. (Translation: The Palm Sunday Procession commences.
(what could be added to improve the description? Answers: Location of event, who sponsored by, who is participating)
English: Arthur Kill from overpass in Staten Island, with Carteret in the background
(What could be changed to improve the description? (Answers: add "tidal strait" after Arthur Kill, "New York" after Staten Island and "New Jersey" after Carteret.) (Thought questions: Does this photo need geocoordinates or not? Should the trailer with graffiti on it be included in the description? )
Photos lacking identification features in Wikipedia