How to create drop shadow on image in So Slider Widget

Hi,

I am trying to create a drop shadow in the SO Slider Widget but cat get it fully working. I have inserted the code:
/* SO Slider Widget */
.sow-slider-base ul.sow-slider-images li.sow-slider-image .sow-slider-image-container .sow-slider-image-wrapper {
box-shadow: rgba(20, 20, 20, 1) 0px 0px 10px 0px;
}

But that only gives a shadow on the sides as the top and bottom of the image are the same height as the slider so I suppose the shadows are there but get cut off.

How do I get a simple drop shadow on my images? Your help would be greatly appreciated!

Thanks,
Stephan