I'm doing this at the moment in some of my quizzes using the authoring tool. I've been using an img tag in the html of my Assessment Results feedbacks to show a conditional image (and text) based on the quiz score. 

Your html in the Assessment Results component should appear like this: <img src="course/en/assets/7c4813405af085965c2db190374dae5408f6e084.gif"> where the file name references your image.

The tricky bit is to generate this filename. There may be a simpler way .. but my method involves creating and then hiding a graphic component in my course that contains the image I want to use in Assessment Results. Create the component with the image, preview it, use inspect element (or right-click save-as) to identify the filename and copy this to use in your img tag. Once you've got the filename, hide the block/component with the image by unchecking Is this available.

Let me know if you need more info.