Do you see this alert?

In order for WhatsApp to use the photos from your Camera Roll or Photo albums, you will need to give the app permission to use the Photos from your iPhone. If you declined permission to use photos, you can grant permission by changing your iPhone Settings:

In iPhone Settings, tap [Privacy]:

Tap [Photos]:

Make sure WhatsApp is set to ON

Open WhatsApp, and you should now be able to use your existing iPhone Photos with WhatsApp.

Cheers,
WhatsApp Support Team