Gallery like a collage, without grid 7.1

I am looking for a way to arrange a gallery more freely without a grid line - like a collage. Sure I can do that and insert single images into a fluid section. But there I have to insert each image separately and the lightbox does not recognize the other images, so you can not switch between the images. 

The whole thing should be like a wall gallery, just a bit looser, with different sizes and spacing (random). I haven't found a solution in squarespace or external plugins yet. Thanks a lot!