Lets say page 629, 630, 645 and 712 all use the same header, you could code
HTML Code:
body.page-id-629 div#myimage,
body.page-id-630 div#myimage,
body.page-id-645 div#myimage,
body.page-id-712 div#myimage {
background-image:url('http://blog.privatelabelfitness.com/wp-content/themes/atahualpa/images/WM_Banner_01.jpg') !important;
position:relative;
margin:0;
padding:0;
height:350px;
}
body.page-id-629 div#imagecontainer,
body.page-id-630 div#imagecontainer,
body.page-id-645 div#imagecontainer,
body.page-id-712 div#imagecontainer {
display:none;
}
you could put all the pages into the one {display:none;}