I just tried removing the access photos permission entirely from Whatsapp on my Android phone. Then, sharing a photo from within the Photos app pops up the permission dialog in Whatsapp. You need to give it at least the "Limited" permission, otherwise it won't process the shared photo.

Having given it that permission, I can share photos from within Whatsapp as well, without going to the Photos app. I'm not sure if the file picker that pops up is a Whatsapp component (meaning the "Limited" permission is essentially unlimited) or if it's a system component. I mean the latter would make sense, but I'm too cynical to believe it works that well.