This could be due to one of many reasons.
1) Make sure that the path to your image is correct. All Web content must reside within your 'public_html' directory to be viewable via the Internet.
2) Make sure you're using the right case (i.e., uppercase or lowercase) when accessing your files. If you’re trying to access a file located at "image/image.gif", and in fact, it is actually called "image/Image.GIF" on your Web server, then the image will not load properly.
3) Make sure that you are uploading your images in binary mode with your FTP program.