Using an email as storage for images is not really a good idea, imo. If you delete the email completely by accident, they will be lost. There are a few other ways you can lose them forever.
If you need the storage space on your iPhone, import them to online storage, such as Google, e.g. As long as you don’t decide to view them offline, they won’t need your device’s storage space, and they will be available when you need them, if you’re connected to a network.
Or import them to your computer, or a hard disk drive.