PNG files in the layerslider

Hi there I would like to introduce some of my products as layers in the frontpage slider. Similar to the demo page where the images of computers, smart phones etc drop in on top of the background image. My question is what format would I need the images in so that they are not simply pictures dropping on top of each other, They would almost need to be icons or transparent PNG files to get the same effect. Hopefully it make sense what I am trying to do. Any help is greatly appreciated.