Checks if your page is using an image expires tag, which specifies a future expiration date for your images. Users browsers will see this tag and cache the image in their browser until the specified date (so that it does not keep re-fetching the unchanged image from your server). This speeds up your site the next time returning visitors arrive at your site and require the same image.
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How do I fix it ?
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:
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"