Those graphics are controlled by the style.cfg. In in mg_themes it's called mg_themes.cfg and the paths are set thus:
$images['tbl_h_l'] = '<table class="roundedtop" width="100%" cellspacing="0" cellpadding="0" border="0"><tr><td align="right" valign="bottom"><img class="topcorners" src="' . $current_template_images . 'tbl_h_l.gif" alt="" /></td>';
$images['tbl_h_c'] = '<td class="roundedhc" width="100%" align="center">';
$images['tbl_h_r'] = '</td><td align="left" valign="bottom"><img class="topcorners" src="' . $current_template_images . 'tbl_h_r.gif" alt="" /></td></tr></table>';
$images['tbl_b_l'] = '';
$images['tbl_b_c'] = '';
$images['tbl_b_r'] = '';
$images['tbl_f_l'] = '<table class="roundedbottom" width="100%" cellspacing="0" cellpadding="0" border="0"><tr><td align="right" valign="top"><img src="' . $current_template_images . 'tbl_f_l.gif" alt="" /></td>';
$images['tbl_f_c'] = '<td class="roundedfc" width="100%" align="center">';
$images['tbl_f_r'] = '</td><td align="left" valign="top"><img src="' . $current_template_images . 'tbl_f_r.gif" alt="" /></td></tr></table>';
It is not wise to start changing the config, but rather keep the original file and folder structure. You would also have to edit theme_info.cfg to set the style path to your new theme if you have called it something else.
Edit: Also be aware that changes to the theme_info.cfg have no effect until you uninstall and re-install the theme.