Double Border Around the Post Images

I’m trying to get a 1px solid border directly on the outside of the image with a color of #424242; then 5px of padding then an outside border around the padding with the same #424242 color. Because when the images have a white background it messes up the way the “frame”look is supposed to be around the photo.

My current CSS is:

.post img {
padding:5px;
border:1px double #424242;
background-color:#fff

You can see how it currently looks at https://www.cisterspeak.com

I’d really appreciate the help.

Thanks.