The red X indicates that the image is not available. This could be because of a number of factors, some common occurences are; bad link (link to image is no longer valid), no permission to view the image, or firewall software blocks your access to the image. Most of these symptoms you cannot remedy on your own. You can ask someone else to check if they can see the image, and if so, you can check your settings, but otherwise, you will be dependant on the site administrator.