There can be a number of reasons that can cause your image not to appear, a quick and easy way to find out why is to check out the HTTP response headers, under X-Cld-Error.
In order to see the X-Cld-Error header in your browser:
1. Open your browser's Developers Tools.
2. In the developer console's 'Network' tab you'll see the erroneous image. Note that you may need to do a 'hard refresh' if the image was already loaded when you opened the console (Command-Shift-R on mac, Ctrl-F5 on Windows). Click on the 'Network' tab to see the response headers.
3. Scroll down the headers to see the X-Cld-Error header. The following is an example of a non existent image: