Images can enhance a message and draw attention to its key components. But, when they are blocked and appear as a big, red X in the recipient’s preview pane (often with the instruction to “right click to download”) they can lose their effectiveness. The reason images are often blocked is to enable users to prohibit unwanted (or inappropriate) images from loading automatically.  Blocking images also protects readers from spammers who use them to verify the email address is real. 

The impact of blocking images comes in multiple forms (such as lower open rates and disabled banner ads which can hurt advertiser supported material), but they all lead to lower deliverability. Like other delivery issues, email marketers can institute practices to help minimize this issue.