If you take no other part of my advice, take this. If you want to be successful at this online dating thing, post a good, recent, smiling photo of yourself. If you can post more than one, so much the better.
Photos: Not Really Optional.
Every single dating site will tell you that posting a photo is optional. Indeed, a little looking around will confirm that there are many profiles without photos. There are a few reasons for this and, unfortunately, none of them are good news. Importantly, sites allow trial users to post profiles, usually without photos, and many sites keep these profiles active for some time. It pads their membership numbers and returns more search results for each zip code. If you look at a profile without a photo, check (if the site allows) for how active it is before writing that person. Of course, there can be many other reasons (see list below) for the lack of a photo, and all of them are bad news.
Which photos draw your attention? Webmasters' experience bears this out: Most dating site searches have a default setting to only show profiles with photos. Also, even if you take the initiative in email, the first thing your prospects are going to do is click through to your profile. If there's no photo, they'll never write you back. To sum up, if you don't post a photo, you've got an uphill battle ahead of you.
Posting a photo is even more important for those seeking men than those seeking women. Men are more visual creatures, and are less apt to read a profile in any depth, and are more apt to email on the basis of a photo alone.
What they'll think if you don't post a photo
You are inactive and therefore not worth contacting.
You're not serious about finding someone.
You're a criminal on the lam.
You're cheating.
You're ugly.
You're lazy.
You're a beautiful, rich celebrity who wants to be loved for who he truly is.
Okay, #7 is a long shot.
Beauty - in the eye of the beholder (and the pro photographer)
One reason you might not post a photo is because you think you're unattractive. First of all, you're probably wrong. You know the media standard of beauty is bullshit. But even if you are butt-ugly, you'll be amazed at what a quality smiling photograph can do for you. Even the best-looking people look dour when they don't smile; if you're not conventionally attractive, a confident, genuine smile can only do you good. If you don't have any high-quality photos you like, have one taken. Isn't it worth it to invest in a professional portrait if it brings you your dream partner?
Post more than one photo if the site allows it. Be creative. Try a shot with your dog, holding your musical instrument, cooking in the kitchen, or riding your motorcycle.
Babes - listen up
If you're an extremely attractive woman, choose one of your less glamorous/sexy shots that is still flattering and still looks like you. Otherwise, you'll be barraged with email. I have glamorous pics, but I put up a regular shot of myself in glasses, no makeup, and I got lots of responses. I can't imagine what would have happened if I had put up something more sexy. I like to hide an important nugget of information in the middle of a profile essay to see if prospects are actually reading the text.
Photo No-nos
Posting an outdated photo. Even if you looked better when you were 20 pounds lighter and five years younger, the second your date sees you in real life, you're going to have a big fat lie and pissed-off person standing between you and your goal of conjugal bliss. Plus, the sexiest thing is confidence, and lying in any way in your profile makes you seem insecure.
Posting an overtly sexual photo. Unless you're on a sex/swinger/kink site, posting a racy photo sends the wrong message. Women will be actively turned off, and men will think you're a hooker or cam girl. Even if you're on a sex site, for goodness' sake, don't just post a close-up of your penis. *shudder* A little can go a long way in terms of suggestiveness.
Posing with your kids. I know they're important to you, but giving them too much prominence in your profile leads folks to think you're looking for a replacement dad or mom, rather than a partner for yourself. Mention them in the profile, then send pics if your prospect expresses interest.
Posting a crappy photo. This sends the message that you don't care enough to do a decent job, which isn't very good advertising for your capability as a partner.
Posting a photo with someone cut out of it. Oh, for goodness' sake.
Posting an outdated photo. Even if you looked better when you were 20 pounds lighter and five years younger, the second your date sees you in real life, you're going to have a big fat lie and pissed-off person standing between you and your goal of conjugal bliss. Plus, the sexiest thing is confidence, and lying in any way in your profile makes you seem insecure.