Resize Retina Sized Ad

[Ocean West]

I have a square ad that was uploaded 2x the size so that it is crisp when for retina displays i am trying to figure how 

to resize the width or height so that on display the image is scaled the proper size?

this is the code that I use to list all the square ads vertically for the sidebar.

$i = 0;
foreach ( new \IPS\Patterns\ActiveRecordIterator( \IPS\Db::i()->select( '*', 'core_advertisements', 'ad_active=1 AND  ad_location=\'fmf_square\'', 'rand()'), 'IPS\core\Advertisement' ) as $ad )
	{
		$i++;
        echo $ad ;
	}