Fallback image and timeout – External Content. Javascript

You can add an onerror handler:

<img
  src="http://example.com/somejpg.jpg"
  onerror="this.onerror = null; this.src="./oops.gif""
/>

Note: Setting onerror to null in the handler, so that the webpage doesn’t crash if oops.gif can’t be loaded for some reason.