Most email programs don't show images right away and leave it up to the recipients to choose whether or not to load images. This is mostly for the security and safety sake of their users. In almost all cases it's simply a setting in the recipient's email client preferences.
If one of your subscribers has complained that they can't see any images in your email, it's best to find out what email program they are using, and walk them through these steps. Most folks handle this on a case-by-case basis, reacting to the prompt on the individual emails. But you can encourage your recipients to set emails from you to always show images.
Here's how to enable images in the more popular email programs: