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.

In order to reduce the number of HTTP requests, you can use the HTTP Expires header to set an expiration time for your images or any other content type. You can add the following lines into your .htaccess file:

<IfModule mod_expires.c>
    ExpiresActive on
 
    ExpiresByType image/jpg "access plus 1 month"
    ExpiresByType image/jpeg "access plus 1 month"
    ExpiresByType image/gif "access plus 1 month"
    ExpiresByType image/png "access plus 1 month"
</IfModule>

Media queries allow you to style elements for specific devices (smartphones, tablets, desktop computers) by using attributes like width, height, resolution, aspect ratio, orientation or color. By using media queries, presentations can be tailored to a specific range of output devices without changing the content itself.
Example:

<link rel="stylesheet" media="screen and (min-width: 480px) and (max-width: 960px)" 
href="480-960.css" />
 
<!-- OR -->
@media screen and (min-width: 480px) and (max-width: 960px) {
    #header {
        display: none;
    }
}

An @media rule specifies the target media types of a set of statements. In the example above, we are specifying the media type screen. The max-width and min-width features are telling the browser that at any screen size larger than 480px, but smaller than 960px, hide any elements with id="header".