In order to pass this test you must add an alt attribute to every <img> tag used into your webpage.
An image with an alternate text specified is inserted using the following HTML line:
<img src="image.png" alt="text_to_describe_your_image">
Remember that the point of alt text is to provide the same functional information that a visual user would see. Search engines, users who disabled images in their browsers and other agents who are unable to see the images on your webpage can read the alt attributes assigned to the image since they cannot view it.
Learn more about optimizing images for SEO.