For events run by the Students' Union we post the photos on our facebook page and we try to post these within a week. Our photographers are often students so sometimes they have other commitments which can cause delays.
If the event was run by a club, society or an external promoter then you would need to contact them direct. These photos would not be published by us.
If you think that you photo was taken but not published then we are sorry but we cannot obtain these later, it is possible the photo was not high enough quality eg out of focus.