Image Source

Justified Gallery displays beautifully! I was wondering if there is a hook to change the image source for thumbnails? Currently it loads all original images from the post, which I’d like to use the medium image. I know some PHP, can precisely measure pixel width of the biggest thumbnails vs size of medium using element inspector, and can re-generate any image/modify the sizes. Any help would be greatly appreciated. 😊

Warm regards,
Josh