Who merges photos from a telegram chat?

Question

Bots

Hello.
There is a chat on common interests in telegrams. In total, there are about 400 users in the chat.
From the chat, photos are constantly merged to the site of the same subject. Either it's a bot or some user.
How can you figure out who is doing it?

Answer 1

What two people know, the pig also knows

Put a watermark with the name of the channel.

Answer 2

Instead of a photo, give a link to a photo with authorization via telegram.
When accessing a photo, put an inconspicuous "watermark" (just a number, for example) - id, which uniquely identifies the telegram user's name and write the id and user name to the database.