Hello,
When you use embedHtml, images must be available through an URL (absolute or relative) and set the downloadImages option to true. You can't use a local path to add images.
If the addImage method is returning a file not found error, then the image is not accesible through PHP or it doesn't exist. We recommend you to run the included examples to test this method and check read access to the images you are adding.
Regards.