Fix box sizing on post images

Fixes #76.

Author: 17cupsofcoffee

sass/_extra.scss

@@ -63,7 +63,8 @@ h6:hover .anchor {
 
 .post-content p img {
     display: block;
+    box-sizing: border-box;
     margin: 16px auto;
     padding: 8px;
     border: 1px solid #e8e8e8;
-}
+}