WhatsApp does not have access to my photos
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's Privacy Settings:
In iPhone Settings, tap Privacy.
Make sure WhatsApp is set to ON.
If WhatsApp is greyed out or does not appear in Privacy settings
This is a rare issue, but happens to a small number of users. Ensure that you do not have any "Restrictions" set in iPhone Settings > General > Restrictions. Otherwise, you will most likely need to restore your phone. This Apple Support Page explains the process: iOS: How to back up and restore your content.
Open WhatsApp, and you should now be able to use your existing iPhone Photos with WhatsApp.
WhatsApp Support Team