OK, I've got to say this: I went through a few pages, so I saw ~100-150 people. It seemed to me that women's pictures tend to (i) show more of the body and (ii) how to say this, had a more "flirty" look.

Now, this is my completely subjective impression. However, it seems to me that the women in such technical conferences tend to send in more glamorous shots, to perhaps cater (subconsciously) to the highly skewed gender ratio. What do you think?

As an aside, I think if you want to get more love (from HN or anywhere else) don't emphasize the "pretty girl" angle because the label "girl" is felt to be demeaning by many people and after all in your site one can select both men and women, so why not say "attractive attendee"?

EDIT: Just thought of this: If you gather a lot of data, you can post results on how speaker/panelist attractiveness affects the number of people attending a session, corrected by the effect of the topic, of course. There are many organizers, I think, who would even pay for such information.