Putting white rectangles in flexgrid

Putting white rectangles in flexgrid

I would like to impliment a flex grid in my solution. If you look at this grid:http://tympanus.net/codrops/2012/11/06/gamma-gallery-a-responsive-image-gallery-experiment/

You can see that it works fine, now what I would like to achieve is put random white squares between the pictures, so no white pictures but white squares/rectangles.

I would load the images from contentitems in cms system so that's why i can't use white images.

If anyone would have an idea how to accomplish this it would be great!