Here are some possible reasons why the Red X may appear, and suggested fixes for each:

 

Problem: The "Show Pictures" setting has been turned off in Internet Explorer.
Solution: In Internet Explorer's Tools menu, click Internet Options, then click the Advanced tab. Make sure the Show Pictures check box is selected under Multimedia, and click OK.

Problem: Your anti-virus or firewall is a little too uppity. Check to see if your anti-virus or firewall software has a privacy setting to "disable web bugs".
Solution: Uncheck or turn it off. (The term "web bug" is a misnomer. They are really just harmless tiny images on a web page or email, sometimes used for tracking purposes.)

Problem: Your Internet Explorer security settings are set too high, preventing you from accessing the web site which hosts the image.
Solution: Reset to the default security settings. In Internet Explorer, click on Tools -> Internet Options -> Security -> Default Level.

Problem: You've installed the Windows XP/SP2 updates and you're being bitten by the "Block images and other external content in HTML e-mail" security option. This is automatically turned ON in Outlook Express after you install Windows XP/SP2.
Solution: Click on Tools -> Options -> Security. Then UNCHECK the "Block images and other external content" check box, and click OK.

Problem: You are the SENDER and others are complaining that they can't see images in emails you sent,
Solution: In Outlook Express, Click Tools -> Options -> Send. Under Mail Sending Format, select HTML, then press the HTML Settings button. Make sure the "Send pictures with messages" box is checked.

Problem: The image is hosted on a website that's busy, temporarily offline or defunct. Or the image file on the website might have been renamed or deleted.
Solution: Try again later or ask the sender to send it as an attachment instead of an inline image.

Problem: The image is hosted on a website, and you're not online.
Solution: Make sure your Internet connection is active and try again.