First of all, sending patient data via WhatsApp is risky. See Is sending patient data via WhatsApp safe?.
By default, all photos you take within WhatsApp appear in your camera roll. You have to remove them manually (see previously listed instructions). You also have to remove them from the conversation.
If you have lots of offending images in conversations, here is how you can clear conversations.
Even more problematic, by default, WhatsApp automatically saves all photos you receive into your phone gallery! On iOS it is easy to change the setting to prevent this. On Android it is ridiculously hard - you have to download another app to do it!
To prevent WhatsApp from putting received photos in your gallery on iOS.
There are the ridiculously hard instructions provided by WhatsApp.