I don't really know why it would be still blocking it then, unless it know it contains something malicious. But you said they're from people you trust though. IMO, I wouldn't classify Win Defender as an Antivirus program, maybe more like an Antispyware. To be honest, it's best to have a major antivirus program installed as well, something like Avast Home.
Have you tried temporarily disabling Win Defender just to test if it is blocking the pics?