404 means the file is not found. If you have already uploaded the file then the name may be misspelled or it is in a different folder.
Other Possible Causes
You may get a 404 error for images because you have Hot Link Protection turned on and the domain is not on the list of authorized domains. If you go to your temporary url and get this error, there may be a problem with the rule set stored in an .htaccess file.
WordPress Note
Are you using WordPress? See the Section on 404 errors after clicking a link in WordPress.