Yeah, this is an intermittent problem for me too. Certain contacts occasionally lose their picture in Messages. Only certain ones - others never show the problem. The photo is in Contacts and shows up properly in Mail for example. This never happens on my iPad Air 2 or MacBook Air. Only my iPhone 6S.
The simplest workaround I've found is to just add a bogus phone number to the contact. That seems force a resync. It doesn't work for any other contact info - you must add or remove a phone number. Unfortunately it doesn't seem to be a permanent fix - I don't know if the other options presented above prevent a recurrence.
No question this is a bug but either Apple is denying it or not putting effort into fixing it. Very annoying. I'm also told they don't read these forums, so likely they don't get this feedback either 😟