I think the best way is customise the PHP code from this file JoomlaFolder/components/com_easyblog/themes/wireframe/blogs/entry/post.cover.php , this is the file render that post cover on the blog entry page.

You can use this following function code to determine whether this user currently view on the mobile device or not.

$isMobile = EB::responsive()->isMobile();

If this variable $isMobile return you 'true' mean this user view this blog entry page on mobile device, so you can render thumbnail image size on mobile device.

For example :

if ($isMobile) {

     echo $post->getImage('thumbnail', true, false, false);

} else {

     echo $post->getImage('large', true, false, false);

}

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