I came across two hidden icon options for the $input function in mIRC 6.21. If you specify a 'c', you will get a Recycle Bin icon, if you specify a 't', you will get a Star icon. These weren't in the help file. Happy scripting.
.. I just tested both of those with 6.16, and specifying t provides the star. I couldn't replicate the "?" with the c though. Good find though. Pretty cool.