You can turn off loading remote images in emails in the App Settings - Privacy section. View
You can choose which emails will show remote images.
Always This will always show remote images
Known Senders This will load remote images for emails from people you know.
S/MIME Signed Emails with a valid S/MIME signature will load remote images.

You can also turn off remote images when using mobile data in the App Settings - Email section, 'Conserve Mobile Data'.

Some emails contain external images that are loaded from a remote web server. This can make emails smaller but the network is used to display the images.
External images can also be used to track when a recipient reads an email that is sent. This can be used for read receipts. View

Turning off remote images will display the email without the images. Then you can tap on the 'Show' button in the top of the email to show the images after opening it.

You can see a list of remote images for an email in the email details screen - 'External images'. View