Clipboard screenshot

The user's clipboard is invisible to you. A small bridge on this machine makes it reachable: connect to its Unix socket and it writes the clipboard's current contents to you and closes. When the clipboard holds a screenshot, those bytes are a PNG (sometimes another image format).

The socket is $HOME/.clipboard.sock ($CLIPBOARD_SOCKET overrides it). Living in the home directory is what makes several users on one machine safe: each login's bridge — or forwarded socket, via RemoteForward /home/%r/.clipboard.sock <local> in ssh config — sits in its own home, so paths cannot collide and a grab can never read (or be fed by) someone else's bridge. Never point CLIPBOARD_SOCKET at another user's socket: that is their clipboard, not the user's.

That gives you a path from "the user copied a picture" to "you are looking at the picture" — the missing step is that image bytes only enter your context by way of a file you Read, so the bytes have to land on disk first, under a name whose extension matches what they actually are.