Adding a drop shadow png to images

Site: https://www.broomeandthekimberley.com.au/2011/03/broome-history-culture/

Hi can some-one advise me how to add a drop shadow image to all the images on my site.

I have tried

img {
background: url(images/image_shadow.png) no-repeat fixed center bottom!important;
border: 1px solid #000!important;
}

the black border shows but not the drop shadow. I am using a child theme of Twenty Ten.

Thanks ??