If you include ONLY physical characteristics as a measure, and create some form of consistent baseline for attractiveness, then I'd be surprised if there was any significant impact on online dating site use on attractiveness.
The same kind of financial circumstances that prevent a lot of people from getting online at all (and thus using an online dating site) are the same circumstances that reduce preventive healthcare, wardrobe budget, and other personal care that typically enhances looks (spa treatments, hairstyling, teeth whitening and hair removal).
That said, what people think of as attractive in a potential mate usually embodies more than the physical – a sunny or even devilish personality, confidence and poise, extroversion and a welcoming/inviting nature.
Not everyone can easily pull this type of attraction off in person, and online it can be even more difficult to fully convey these qualities even if they exist in abundance "offline".
I think the fairest thing to say is that it's very difficult to "be your most attractive" online – though breaking the ice and reducing tensions/expectations via an online dating service CAN increase attraction/comfort during subsequent in-person meetings.