Yes. Be upfront about what you are and about what you seek, especially if it is a dealbreaker. Not in the text of your profile (it is a turn-off to read: If you’re chubby, stay away" ) but definitely in your search preferences. Don’t correspond with anyone who doesn’t offer a photo after three e-mails, and explain why.
With online dating, the golden rule (which you’ve learnt the hard way here) is not to get emotionally attached until you’ve seen at least 4-5 pictures, recent, full-lighted, both close-up and full-length, from different angles. It may seem superficial to ask that from prospective dates, but it really, really can save a lot of time, heartache and wasted emotional energy, for both partners. It is perhaps one of the top-three mistakes with online-dating. A mistake made because everyone, at first, does not want to come across as superficial, and also because everyone assumes, at first, they can’t afford to be picky.
Now, as to the wording: what would have been wrong with: “You look great (insert: nice eyes/hair/smile, anything you can say truthfully) but you are really not my type.” The whole idea is for her to attribute the rejection to taste, and taste differs. Some men like blondes, some brunettes, that kind of thing. Blonde isn’t better or worse then brunette.
It is difficult, but don’t let her corner you into saying what, exactly, you dislike about her. Some people stupidly ask questions, even when the answers will only hurt them, and she might do the same. Don’t play along.
Instead say: "What’s the point of saying what I don’t like about you? I’m just not attracted, now that I see you. Believe me, if I could change that, I would, because I really like you. But I know from experience I can’t change who I am or who you are, and believe me, that is my loss. And I’m not going to say anymore about that, even if you ask, because that won’t do any good. "
Firm but tactful. IMHO.