I would recommend not commenting on women's photos. Personally, I hate it and don't respond when I get those. It feels kind of creepy and superficial. Instead of talking about her appearance, talk about things she has written in her profile and things you might have in common. Given the huge flood of messages women receive on these sites, it would be a full-time job to respond to the people you aren't into. Women only tend to respond to the people hey are actually interested in. It's not rude to ignore a stranger you're not into who has commented on your appearance. It's what women do every day when we walk down the street. Most women are looking for someone who is going to stand out by saying something deeper and more engaging and not approach them like a pretty object.