Here are some things to try:
Some web-based email services (like Gmail) automatically block images in the body of an email until the recipient approves them. You should instruct recipients to click the "Display Images" (or similarly named) button in their mail viewer.
Make sure your message isn't too big. Many mail servers cap the size of email messages at 10 MB. You can check the size of your message by looking at the bottom status bar in the Message window.
If you have attached images to your email, make sure that they are in a format the recipient's mail viewer can read. GIF, JPEG, PDF, and PNG are typically safe formats to use.