This probably isn't possible because of privacy settings, but I was wondering if there's a way to find someone's friend ID, if there's no link to their profile? Some people have their privacy settings set so that no one can find them in a search except those that are on their friend's list and when you see comments they've posted on other people's profiles, there's no link to their profile. Is there a way to find their friend ID?

They don't have a profile picture. Their picture is the question mark because they haven't set a default and when you right-click on that, the url is always http://static.ak.facebook.com//pics/d_default.gif
If there is a link to send them a message or view their friends, you could see it.
right click on their profile picture and their friend id will be in there. example:
profile.ak.facebook.com/profile5/260/55/sXXXXX_8755.jpg
(xxxx is their friend id)