When I browse my site on the Internet, no image is displayed. I get a red "X" instead, whereas everything is normal when I test my website on my local PC.

This problem is well known : carefully check your image file names, specifically lower and upper case letters. For instance, "image.jpg" is different from "Image.JPG".

This is usually masked by Windows (which does not care enough about letter case), but is revealed by case sensitive operating systems like Unix. Your Internet Service Provider's machine is likely a Unix machine.

Another cause : images were not transferred by FTP (in this case, check for instance that the "transfer only recent modified files" optimisation is not active : the images you use in your site may have earlier dates).