@Abbbnm: Have you deleted the photo from WhatsApp, as well as your Gallery? If you head to WhatsApp > Tap the 3 dots in the top right of the landing page > Settings > Storage and data > Media auto-download, then you can tap on the available connection categories, and untick 'Photos' to stop them from automatically downloading to your Gallery. As per the above, if you delete the photo from WhatsApp itself, then this should resolve the issue (Open a chat > Tap on the relevant photo > tap the 3 dots in the top right > Delete).
