Whatsapp is the owner of everything that is sent. This can be read in the Terms of Service. If you send a photo Whatsapp is the owner. In theory this would mean that they could resell it to advertisers or anyone who has an interest in pictures.
But this does not seem to happen because there are no messages from known.
This is somewhat incorrect. It is clearly given in the Terms of Services that we’re gving WhatsApp “a worldwide, non-exclusive, royalty-free, sublicensable, and transferable license to use, reproduce, distribute, create derivative works of, display, and perform the information” which we give to them. They DO NOT have the ownership of the data we share including texts and images. They can only use this data for the “limited purpose of operating” and providing their Services. They cannot sell the messages and earn profits from it.
Text:
“In order to operate and provide our Services, you grant WhatsApp a worldwide, non-exclusive, royalty-free, sublicensable, and transferable license to use, reproduce, distribute, create derivative works of, display, and perform the information (including the content) that you upload, submit, store, send, or receive on or through our Services. The rights you grant in this license are for the limited purpose of operating and providing our Services (such as to allow us to display your profile picture and status message, transmit your messages, store your undelivered messages on our servers for up to 30 days as we try to deliver them, and otherwise as described in our Privacy Policy).”