Emails can contain offensive images or can contain invisible images that tell the sender that you have opened that email (and thus verify that your email address is active). To prevent spammers from subjecting you to these images, you may wish to prevent all images from loading until you are satisfied that they are safe to view. This is one of the many ways Yahoo! Mail protects you from spam.

By enabling the option to "Block HTML graphics in email messages from being downloaded," all remotely stored images will be replaced with this image . To view the message with the original images, click the "Show HTML Graphics" link at the bottom of the message.

Why do I still see images in Yahoo! Mail?

Images that are sent as attachments will be shown in the attachments area, even with this option turned on. Also note that this option will not prevent images outside of the email messages from loading. Navigational, informational, and advertising images will still be loaded.