Can't I just use the camera in iMessage

No. It is near impossible to not breach privacy regulations doing this. Below we list six ways privacy regulation violations often occur.

1. There is no documentation of consent.
2. Patient date travels internationally through Apple's servers.
3. You're storing clinical photos alongside personal content.
4. De-identifying photos isn't/can't be done properly.
5. Clinical photos may be accessible if you lose your phone.
6. There is a question over the encryption used.

For a detailed discussion, please see the "Can't I just use the default camera app to take clinical photos?" FAQ. Also see the "Is sending patient data via iMessage safe?" FAQ.