The Warning: Cannot modify header information - headers already sent message is not related to the smush.it plugin. You might have encountered this when saving the options, there, but it could be anything! It is a general PHP error, indicating that there's probably something wrong in a PHP file in your istallation.
More info about this can be found here: http://stackoverflow.com/questions/8028957/headers-already-sent-by-php/8028987#8028987
From personal experience, this usually means that the wp-config.php file has one of the following:
1. A space or something else before the opening
<?php tag of the file.
2. a php closing tag
?> at the end of the file, followed by other characters, a blank line or a space
If it's not in your wp-config.php file check your theme's functions.php for the same things.
If that doesn't solve the issue, start deactivating your plugins until you figure out which one is causing this error.
Once you figure this out, you'll have to open up its files and check the beginning and the end of each file to make sure that there's nothing before the
<?php and that the files don't have any characters after the closing
?> tag. Preferably, the closing
?> tag should be omitted altogether.
I hope that helps!