Here is the scenario. You arrange to meet someone you met online. You are excited about it. He or she is excited about it. Your date sends you a few pictures before the date and you discover that this person just isn't your type. What do you do?
First off, stop feeling bad for being shallow. There is nothing wrong with you. The importance of appearance should not be underestimated. We are human, and it is natural to want to be physically attracted to someone. I've actually never understood how someone can say looks aren't important and that believing otherwise is shallow. Why is it any better to like someone because they are funny, or successful, or a fantastic kisser? You have to like a person for everything they are, and appearance is a big part of that.
Next, just call the person and cancel the date. Don't write an e-mail. Call the person out of respect. It is a lot harder to do over the phone than it is to write an e-mail, but nobody ever said that it would be easy. Getting the news in any way is bad, but getting the same news over e-mail just plain sucks. Just politely tell the person the truth. That doesn't mean to go into every last detail about why you found them unattractive. You should simply say that you have in fact decided the person was not your time and that you don't wish to waste that person's type going on a date when you know nothing will materialize in terms of a relationship.
Under no circumstance should you just go on the date and then ignore the person. That is the absolute worse thing you can do. Nobody wants you to go on a pity date, much less the person on the date. Nobody wants to waste their time going on a date when the other person knows that this will be the only date. So just do the right thing, call and cancel. Be honest, and be respectful.