Either the image was not uploaded, and therefore does not exist on the server.
Your path to the image is incorrect. You may have put it in a different folder, or named it using an uppercase but created a link to it using lowercase.
Some of our servers run on Linux. They don't like spaces in filenames and they treat capitals and lowercase letters as completely different letters. It is best to name files in all lowercase.
You will see a red X whenever the server cannot find an image. Right click on the red X and choose Properties. This will show you the link to the image so you can track down the problem.
The cause may also be from not uploading the image correctly. When using FTP to upload images, be sure to upload the image in binary mode instead of ASCII mode.