Explanation/Reference:
Using Adaptive Images
You can use media queries to select an image resource to display in the page. However, every resource that uses a media query to conditionalize its use is downloaded to the client. The media query merely determines whether the downloaded resource is displayed.
For large resources such as images, downloading all resources is not an efficient use of the client's data pipeline. To selectively download resources, use javascript to initiate the resource request after the media queries perform the selection.
References: https://docs.adobe.com/docs/en/aem/6-0/develop/mobile/responsive.html