which code I will must insert to header to add a image with external link and customize (margin & padding) to set it to the right place...

Which class I will must use, or can I do it without it?
I would like to add it under the logo.

Here's a some code(logo)

PHP Code:
/*---------HEADER STUFF-----------*/
a#logo  {width:100%; background: #000; background:rgba(0,0,0,.85); display: block; clear: right; padding: 80px 0; text-align: center; color: #fff; margin-bottom: 0px;}
a#logo:hover {text-decoration: none;}
a#logo img {max-width: 80%;}