There are often 17 ways to do any particular action with Drupal - here is #18 on the list of "How do I display related content on this node's page?" You'll be using the Views Attach module and a nodereference field.
In this example, you want to relate a photo to an event. Inside the photo's content type, create a nodereference field that is able to reference the event content type. When a user is adding a new photo, she will be able to choose the related event if applicable.