Painting over faces, or using mosaic blur techniques, will prevent any possibility of reversing the effect.

You also want to remove any and all metadata from your images.

While there are a plethora of apps that will help blur or cover faces and remove metadata for both iOS and Android devices (some of which I mention below), there are ways you can do both without using a third-party app.

On iOS, open Photos, tap on your photo and select the Edit option (in the top right corner).

It’s not quite as easy using an Android phone.

Android also has a native markup tool — in the Photos app, select the photo, tap on the Edit tool (second from the left on the bottom) and choose Markup (second from the right on the bottom).

Recently, there have been a plethora of apps that will help hide faces and remove metadata for both iOS and Android devices.

If you already use apps to edit and enhance photos, you might be able to use those to blur as well

If you want to take photos now and remove the meta later, you can use the apps mentioned above or photo apps like Halide (iOS) and Snapseed (Android)

For me, I would take photos, edit them in the phone Photo app, take a screenshot — and then delete the originals