We use WhatsApp, you can delete it if you change your mind (or notice its the wrong person) or select for the photo to be deleted once viewed if you are worried someone else could stumble on the chat but it still remains to triple check it’s the right person before sending!
Me and the husband also use WhatsApp I just always double-check it’s my husband that I am sending it to. I don’t want to make the mistake my husband made, he sent a message for me to our teenage son our son is at the age where he understands these things and he just found it funny and just told his dad that he needs to be more careful
Whatsapp
I’ve been lucky not to send any photos or messages to anyone apart from my wife. BUT almost sent a photo to a group chat once which I’m in with my wife & her girly friends
Signal, Telegram and WhatsApp are all encrypted. I would never use a non-encrypted app.
A tip from my divorced lawyer friend - messages and emails that are not encrypted are discoverable by the courts. They are also easy to hack.