Image blocking is the process by which most desktop clients and mobile email apps automatically disable the images in HTML emails for security reasons.
In this case images are replaced by empty boxes which display ALT texts or a little red cross when the email is not optimized for image blocking.
Image blocking is an issue for email marketers as it lessens email impact and effectiveness and flaws email opening measurement.
Image blocking may be prevented with whitelisting and by enticing receivers to add the sender email address to their address book. The latter is also a way to ensure delivery.
To limit impact of image blocking email marketers use ALT text and text content.
In the example below alt text is in blue: