Thumbnail proportions different to fullscreen image in Masonry Gallery

Hi Staff,
great theme. The best theme in flexibilty and graphic interface!

I have an “advanced” question: I’m using a Masonry Gallery to show some photos. I would like to have an irregular disposition of thumbnails by using Flexible Masonry Gallery like in this example: http://kriesi.at/themes/enfold/shortcodes/masonry-gallery/ (with different proportions)
But my large images are all horizontal in 4:3 format. So if I use the Flexible masonry gallery I see a perfect grid.

I can’t cut the large image, because the zoomed pictures have to be like the photographer shooted it, but i would like to cut only thumbnails to have an irregulare disposition… is it possible? How can I do?!