Because it downloads a picture from a server, this message may identify your inbox as a working destination — an effective place to send more spam. Outlook automatically blocks such pictures but lets you decide whether you want to download and so give information to the sender.
Pictures have the potential to tell junk senders that yours is a valid e-mail address. To help protect your privacy, Outlook purposely blocks certain types of pictures.
And there's another thing. Some junk contains pictures that may irritate or even offend you (unwanted photos or advertisements may appear in the reading or message pane).
What gets blocked (and what doesn't)
Pictures are blocked only if they're downloaded to your computer from an outside computer. Organizations often link pictures kept on their own computers to newsletters or ads in e-mail. And that can be a good idea, because that way pictures can be updated and changed in the computer where they're kept.
When most people create e-mail with pictures, the pictures are attached directly to the message. They aren't downloaded from an outside computer, and so they don't tell anyone that you opened the message. Pictures that you attach to a message, or that your friend attaches to a message and sends to you, would not be blocked by this feature of Outlook.