It seems that sending a photo from the gallery to 10 Whatsapp accounts uploads the photo simultaneously to the 10 accounts from the phone.
Phone gallery -> account 1
                       -> account 2
                       …
                       -> account 10
But, sending the photo to one account and then forward it to the other 9 accounts seems to happen only on the server, i.e. no further uploads required.
Phone gallery -> account 1 —-forward—> account 2,3,4,…10
Does anyone know if this assumption is correct? How does everyone do it?





Whatsapp will also degrade the quality of the photo to accommodate smaller file size/storage.
I would upload onto something like google photos and then share that album link.