This function also adds a parameter to <input type="image"> fields!
Example:
This code:
<?
output_add_rewrite_var ('var','value');
echo '<form action="" method="post">
<input type="image" src="image.jpg" alt="go">
</form>';
?>
will output something like this:
<form action="" method="post">
<input type="hidden" name="var" value="value">
<input type="image" src="image.jpg?var=value" alt="go">
</form>