Have you also wondered why Sitecore adds a “hash=” property to the image query string?
The hash key will protect your images from being scaled by others than your own server. The image above will only render if the hash key matches the width parameter:
The media request protection feature restricts media URLs that contain dynamic image-scaling parameters, so that only server-generated requests are processed. This ensures that the server only spends resources and disk space on valid image-scaling requests.
Sitecore, Protect media requests
This protects your server from using resources scaling images, if anyone tries to get an image from your server in another size. If the hash doesn’t match, the image is not scaled.
The feature can be disabled. In App_config/Include/Sitecore.Media.RequestProtection.config, Set Media.RequestProtection.Enabled to false:
<setting name="Media.RequestProtection.Enabled" value="false" />
MORE TO READ: