The Threema Web application.
I just experimented a bit with different browsers however it works fine for me with Firefox 88.0.1 and Chrome 90.0.4. If you're on windows you can use the clipboard history (Win+V) to validate the picture is actually in your clipboard. If it for some reason still doesn't work at all I suggest writing a Threema Web Bug Report via GitHub or waiting for the Threema Desktop Apps, however they don't have a launch date yet (as far as I know). If you need this feature right away I suggest trying a different browser like Google Chrome or Ungoogled Chromium if you don't want any Google services.
Threema: Three Strikes, You’re Out
4 projects | reddit.com/r/Threema | 8 Nov 2021
Started working on a Dracula UI theme today
1 project | reddit.com/r/Threema | 6 May 2021
Is it time to leave WhatsApp – and is Signal the answer?
1 project | reddit.com/r/privacy | 25 Jan 2021
Can you build IRC Chats between users into a website chat frame
1 project | reddit.com/r/irc | 7 Aug 2021
Lispers active anywhere else with a better topical discussion tool?
1 project | reddit.com/r/lisp | 25 Jun 2021