We recommend using a dedicated app like PicSafe where photos never enter your device photo gallery. If you don't want to use PicSafe, you will need to remove medical photos from your phone as soon as you take them. This is much more involved that most people realize because sometimes "delete" does not mean what you think it does!
To delete a photo on iOS:
To delete a photo on Android:
Unfortunately, on Android, messages and photos are not permanently deleted. When files are “deleted” on Android, really all that happens is the operating system labels the disk space that stores the file as free space. The data is still there, and off-the-shelf data recovery tools can easily recover it. What do we do about this? There are numerous free apps in the Google Play Store, such as Secure Eraser, that will allow you to delete these files properly. Tools like this write over the so-called free space with random data. You should probably install and use a tool like this to remove all “deleted" data.
If you want to remove all existing photos from your phone, please see the How do I prevent my photos from being backed up overseas? FAQ.