Mandatory uploading a photo for negative Rating is a good idea.
But my final proposal would be:
1st step: getting an ID
2nd step: uploading a picture (sender) -> letter in progress
Without uploading a picture, a letter will not get status "in progress".
3rd step: letter will be registered and receiver can upload a photo, too. (optional)
Danubius, I am no IT-professional. I don't know how much effort this would be.
But the result would be: We will see each exchange with a photo!!!