There can be multiple causes for this, but first verify that you do not have any network or firewall issues (software or hardware based) as these are common sources of problems.
You have our Flood Control plugin installed, and Bandwidth Theft Protection is turned on.
If images appear to be blocked for everyone (and/or you receive a 403 when trying to view them), then you have misconfigured or incompletely configured, the Bandwidth Theft Protection section. Typically you haven't inputed all the possible domain names under which your site can be accessed (the plugin tries to figure at initial installation what domains and IPs you have, but it isn't aways complete). From the instructions: "Set the "Allowed Referer Domains" to all domain variations on your site (www.yoursite.com, yoursite.com and IPs) or permitted off-site addresses, one per line."
If only some groups or individuals experience this, then see the original suggestion above regarding firewalls. Some personal and corporate security products, in an attempt to anonymize the surfing of their users, prevent Web browsers from transmitting the URL of the current Web page when requesting a graphic or new document. Bandwidth Theft Control will not display them if it cannot verify that a reader is coming from any of the web sites it is protecting. The security product should be adjusted to permit referrers for your site. If not, you may select the "Permit Blank Referrers" option, however be aware that this loosens somewhat the ability of the plugin to protect your resources.
Your images paths, as set in the General Control panel, are incorrect. Verify and adjust as necessary.
You have uploaded custom images directly into image folders via the operating system, bypassing WebCrossing's FTP. In this case WebCrossing doesn't know the files exist yet. Select Resynch to host filesystem. (Only required when filesystem changes are made outside of this server.) from the Site Managment section of the Control Panel.