Ive been reading through the iMessage documentation and I haven't been able to come across a clear answer to whether or not its possible for an iMessage app to be alerted to and grab specific content from a message. For example being alerted that a someone sent a message with a photo or link and then being able to grab that photo or link and place it in the iMessage app.
This is not possible. You only get access to messages sent via your application. Apple is very strict on privacy and there would be no way to ensure privacy if apps had access to message content.