In WhatsApp Quickly delete all photos or messsage from a certain chat
It’s possible to delete all the messages, photos, or videos, GIFs, contacts, locations, voice messages, documents and stickers in one chat. For example, you can delete all the messages in a specific chat or group chat, but leave all the photos.
iOS & Android: Settings > Storage and Data > Manage Storage > Select Contact > Manage > Select > Tick the boxes next to what you want to delete (videos or images) > Tap on the bin to delete.
It’s possible to sort by which files are taking up the most room, the newest and the oldest if you tap the circle in the bottom left corner after you tap on the contact.