Light Page randomise images

I’m using Light Page stack and would like to drop some still images into the stack and have them randomised like a Pinterest page. I’m finding that they will only organise themselves in a responsive flow, one image under the other. I have tried mixing up the size of the images, hoping that a small image would slide into the space next to a larger one but it doesn’t work. Even when I select a landscape mode with very small images they still organise one below the other.

I’m using the Vingnette Theme in Iris