Border around post images not header

I’m trying to get the following border around my post images without affecting my header but its not working:

.post img.left {
display: block;
padding: 3px;
margin: 0 auto 1.5em auto;
border: 3px double #bbb;
float: none;
clear: both;
}

I can get a simple border around the images, but it also affects the header. I’d like to have the grey border around my post images without it wrapping around the header image too. The only way its been working for me is if I put in the simple img code in the /*Header*/ section which applies the code to my header image too.

https://www.cisterspeak.com

I would really appreciate the help.

Thanks.