Why do images in some messages not load?

An image in a message can be a tracking pixel: loading it tells the sender that you opened their mail, roughly where you were and what you read it on.

So remote images do not load by default. When you choose to load them, they come through our servers rather than yours, so the sender sees our address and not yours, and learns nothing about your device.

Attachments are different — they are already in the message and nothing is fetched to show them.