Hi Michael,
Go to public_html>wp-contect>themes>clean-retina-pro-2>images, Just add your both two images an write the css code in style.css
You also have to customise the code. Adding only the css and images will not show any image. You need to set the default values in clean retina pro -> library -> admin -> theme-options.php and cleanretina-themeoptions-defaults.php, This is for the admin section.
To display it in front end then you need to customise the code inside clean retina pro -> library -> structure -> header-extension.php, Customise your code in this page to display it in the front end.
Thank you!