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, check the box Preserve log. Refresh the page and 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: