Add WhatsApp images to Google Photo album in the cloud
You have to manually change the settings for it to be backed up.

In the Google Photos app:


go to the Albums tab 
locate the PHOTOS ON DEVICE section
scroll to find the WhatsApp Images folder 
when you click to open it, on top, right after the title, there is a Back up &amp; sync switch -> click to activate it.


If you also want to back up the videos, there's a separate folder for that, you have to activate the switch for it too - and for any other local folder you might want to have in the cloud.