This could be for many reasons.
First, you'll need to make sure the links in your <img> tags go to pictures that exist within your account.
Next, make sure that if you have Hotlink Protection enabled, your domain name is allowed to view the protected extensions.
Next, make sure the permissions on your pictures allow them to be read by web users. i.e. 644 or something similar. You may want to search for unix file permissions or image file permissions on the search engine of your choice.
These are some common reasons why pictures may not work but not a complete list for every situation. You may wish to contact a web developer or your web host for further assistance.
