Example one

As you're sorting these, the image placeholder used by jQuery Sortables has no SRC and therefore cannot have a width/height applied to it. Note that on Chrome, this example will work fine. Firefox is the issue here. I haven't tried any other browsers.

Example 2

As you're sorting these, you'll notice that it correctly leaves space open underneath the element you're sorting which is just as big as the element itself. This is how example 1 should operate but can't due to how empty images are handled by some browsers.