As kobra says, something you have done, probably with your template, has replaced all the standard Zen Cart center column content structure with (apparently) a collection of unlabeled tables and some divs.
To put your shadow effect in, you can (in tpl_main_page.php) wrap two divs arount the center area, from just above the "<td valign="top">" above the breadcrumbs to just above the "if(COLUMN_RIGHT_STATUS..." line.
In the first one you can put your left side image, something like a 5x5px bite of shadow, with background-position: top left; and background-repeat: repeat-y;.
In the second one you can put the whole top shadow image with the top left corner-turning, say 5px h x 1000px w, with background-position: top left; and background-repeat: no-repeat;.
Or, if you can be content with a standard edge, use one div with border-style: inset, none, none, inset; and color/width as desired.
PS - You can set $flag_disable_right = true in tpl_main_page.php and eliminate even the shred of html output about it.