off the top of my head no idea... not that that is a lot of help...
though I thought I would have a go...
open viewtopic.php and find
$quick_quote = '<input> ';
replace with
$quick_quote = '<img src="templates/YOUR-TEMPLATE/images/lang_english/YOUR-IMAGE"' . $lang['Quick_quote'] . '" onClick="addquote(' . $postrow[$i]['post_id'] . ');" /> ';
remember to change the code which I have highlighted
