Dating photographs and postcards is often very difficult. Sometimes you get lucky and a photograph has the date written on it or there is a surviving archive where you can match the postcard number to a year; but mostly one is reduced to using posting date to determine that the picture was taken before that date.

If a definite date is given then that date is accurate as far as I know……….. if a date is given followed by p’k then that is the earliest postmarked date I have found for that image.
If I can only date a photograph by clothes, vehicles or buildings then it will be marked circa or pre depending on which is the most reliable. For example, if there is a building shown that I know was built or changed in a particular year and there are no other clues I will mark it pre. If I am relying on clothes it will be circa.