Taking the odd photo that is confidential and shouldn't be synced or posted anywhere I use Google Photos to backup all photos. For my camera app I use OpenCamera. On the occasion I take a picture of something confidential (for work or personal use). What is the easiest/best way to to take a picture and have it only stored locally on the phone (even better if password protected)?

I tried putting the phone in airplane mode but this doesn't work so well as I need to use the internet before I can delete the picture. 

I tried turning off google photos temporarily but this is tedious and I think there's other apps that automatically get the picture, for example Facebook was asking me if I wanted to post the new picture and gave a preview.
Use Signal Messenger. It has the option to send notes to self.