If you used an FTP program to do it, did you make sure that your images went to the server as binary files, rather than the ASCII file type that is used for html pages, test, and scripts?
Good FTP programs will detect the correct file types automatically, and will upload in the correct manner, but on some programs you must set the file type yourself.
Another common error is to design the site on your computer and insert images from your hard drive without placing them in a special images folder within your web. That means that all the image file paths still point to your hard drive location, and the images themselves aren't on the server. You must make sure that copies of all images are placed in the web structure on the server - preferably in a separate images folder.