http://y7yea4pmqqtznb33qiugvysyn2bob5v62e4pvoadoibrwkq3tsddjeyd.onion/ur/module/how-to-use-signal
In a chat, select a photo or video you want to send. Tap the infinity icon above the text box to switch it to the view once mode. The symbol should now be a "1" and the text box will read "View Once Media." Now, when you send the photo or video, the recipient can only view that content one time, but keep in mind they can still take a screenshot of it.