1. Sometimes it’s working with images from the media library, sometimes not. I don’t know why, but it’s a general WordPress problem. Better is to upload new images, then you can be sure it’s working.
2. No, at the moment it’s not possible, but if you know how to change PHP code, you can find the shortcode in the 7league folder -> shortcodes.php
3. No, WordPress does the job for you. It’s better to work with the “normal” sizes of the images for the big image on the top (when you click an image…)