Just add delegate: 'a' to your jquery function and make sure the img tag contains the full res image while the href contains the thumbnail.

$('.popupgallery').magnificPopup({ 
    type: 'image', 
    delegate: 'a', 
    closeOnContentClick: true, 
    image: { 
        verticalFit: false 
    } 
}); 

You may add just:

$("#gallery").justifiedGallery({
        'usedSuffix': 'lt240',
        'justifyLastRow': false,
        'rowHeight': 120,
        'fixedHeight': false,
        'captions': false,
        'margins': 1
    }).magnificPopup({type:'image', delegate: '.gallery-image'});