Never posted here, but I was looking for a solution to pass values with an image submit button for the EXACT same reason. What I came up with is a little different and is probably what you were looking for. And so, though this post is really old.. I'm posting so anyone in the future will have a solution. It seems that if you use brackets as the name, it'll set it as a key in the array, which is quite useful for doing something such as:
HTML/PHP Shortcode:
<input type="image" name="remove[<?=$product_key?>]" src="/remove_item.gif" value="<?=$product_key?>">
Where the key could be any sort of Product/Item ID, whatever..
PHP:
if($remove) {
// use key as the name
$remove_item = key($remove);
//remove item from session
unset($_SESSION["cart"][$remove_item]);
}
Hope this is helpful to someone out there!
Jeremy