First visit the page where you want post number 1 and view the source code of the page. Now do a 'find' on '<body' and you will see something like this
HTML Code:
<body class="page page-id-7 page-template page-template-default">
HTML Code:
<body class="single single-post postid-1976 single-format-standard">
HTML Code:
<body class="archive tag tag-soups tag-55">
From the 'page' page you see page-id-7
From the 'post' page you see postid-1976
From the 'category' page you see tag-soups
(the items in red will vary on your site)
These are the unique items you will use in your CSS Selector. Now you just build the CSS Selector and CSS rules to use the image you want and place it in the CSS Inserts section of ATO>Add HTML/CSS Inserts.
HTML Code:
body.page-id-7 div#imagecontainer { background-image: url(http://yourdomain.com/wordpress/wp-content/uploads/page-id-7.jpg) !important;}
HTML Code:
body.postid-1976 div#imagecontainer { background-image: url(http://yourdomain.com/wordpress/wp-content/uploads/postid-1976.jpg) !important;}
HTML Code:
body.tag-soups div#imagecontainer { background-image: url(http://yourdomain.com/wordpress/wp-content/uploads/tag-soups.jpg) !important;}
HTML Code:
body.page-id-7 div#imagecontainer { display:none;}