to use a hard coded page_id is a solution, but never the best one because the ID of the start page will change if another language is selected... and some more reasons too.
It's more sure an flexible to get the ID from database / page-tree like the core and SM2 do it also.
<?php
$iStartPage = intval(show_menu2(1, SM2_ROOT, SM2_START, SM2_ALL|SM2_NUMCLASS|SM2_BUFFER,
'[if(class==menu-0 && class==menu-first){[page_id]}]', '', '', '', '', ''));
based on this you can select your image
<?php
if( PAGE_ID == $iStartPage ) {
echo '<img src="..." />';
}else {
echo '<img src="..." />';
}
this should create your menu:
<?php
$sMenu = show_menu2(1, SM2_ROOT, SM2_ALL, SM2_TRIM|SM2_PRETTY|SM2_BUFFER,
'[li][if(class==menu-expand){[MENU_TITLE]}else{[a][MENU_TITLE]</a>}]',
'</li>',false, false,false, false);