The first part is make sure your browser.display.show_image_placeholders variable is set to true in your config file. To do this type about:config in the address bar of the browser, hit enter, and then do a search for this variable. A double click will change the value of the variable.

If you still can’t see the icon despite the fact that browser.display.show_image_placeholders is set to true then you need to follow this steps:

• Go to the Firefox profile folder.
• Go into the user folder found there.
• Go into the chrome. folder
• Duplicate the file userContent-example.css and save it as userContent.css
• Open a text editor and paste the following code:

  /*
   * Show image placeholders
   */
  @-moz-document url-prefix(http), url-prefix(file) {
    img:-moz-broken{
      -moz-force-broken-image-icon:1;
      width:24px;
      height:24px;
    }
  }
  

• Save the file and restart Firefox

On Mac Os X the path to the css file would look something like this:
/Users//Library/Application\ Support/Firefox/Profiles/ /chrome/userContent-example.css

I hope this had solved your problem.