Css doesnt provide the feature of double border . its just a trick.
The code would look something like this .

CODE

#content img {

border: 4px solid #159AC7; /* gives outer border of white color*/
padding:3px;
background:#000; /* gives black color of padding 3px

}

Dual (double ) border image

compatibilty : IE 6,7,8,9, mozilla , chrome.
hope it helps you