Many times, a users email client will block images until they take action to display images. This is usually true from people that have not added a specific email address to their list of safe senders.
Try right clicking on an image and see if it has an option to display images or add to safe sender list. If you are using a web based email such as Hotmail, Yahoo Mail or GMail, then see if there is an option near the top of the email to display images.
Ted