Why do I have multiple pictures for some contacts?

If I go to Contacts and then open the entry for someone then I (often) see a picture of that person. If I go to Messages and open a conversation with that same person and then click Details, the picture shown is often a different picture of the same person. (Sometimes there is no picture, but it isn't as if the picture is "wrong".) Why is this? It *appears* that Messages is using the information from Contacts, because I see birthdays, parents, etc., which I enter in Contacts, so I would expect it to use the same picture, too. Thanks to anyone who can shed some light on this.