[kobayashi]

Same way iMessage does

[dmix]

The same way Signal does is probably a better example.

[microtonal]

Keep in mind that Whatsapp still supports pretty old devices (Nokia S40/60), where you may not want to encrypt every image N times.

Edit: remove some stuff.

[kccqzy]

Actually iMessage doesn't encrypt each image N times. It encrypts once, upload it to a server, then encrypts the image encryption key N times.