Images are disabled by default in many of the major e-mail clients — AOL 9, Outlook 2003, Outlook 2007, Gmail and others. When recipients open a message with images in one of these clients, they’ll see red x’s, warning messages, grey boxes… or simply nothing at all.
If critical branding information is in those images, readers may not even be able to easily identify the source of the message and may believe that it’s spam.
Even if the information in the image itself isn’t critical, be aware that the warning messages in Outlook 2003 and Outlook 2007 can distort your message to the point where it’s illegible. Provide the image size in the coding, and the message will be legible even if the image doesn’t load properly.