If the world were honest, then one picture would be enough. However, the world is not honest. The Internet is not honest in particular. Unfortunately for us, it is not enforceable to keep people from lying on the Internet. By now, we've all received some sort of fraudulent email, advertisement, or instant message making bold promises that really are just good to be true. One photo can be taken from anywhere else on the internet and posted on a profile. In fact, at least 20% of all dating site "females" are fake photos. Asking for multiple photos is not too much to ask when everyone has a digital camera on their phone.
One picture on a dating site is generally not considered adequate. This is because a single picture can often be deceptive. For example, if the photo is a close up of the face, then one does not really get an idea of physique (e.g. height and weight). Conversely, a full body photo may not give a person an adequate look at the face. Therefore it is preferable to put multiple photos, in order to be honest and avoid later disappointment.
Thinking back to my on-line dating experience I like to see a two to five pictures because I feel you need at least one portrait and at least one photo that shows the whole body. Since people on the Internet often like to misrepresent themselves, you have to consider this when creating a profile.
No, I do not think that one picture would be good enough for someone to put up on a dating site. I think that you need several pictures, showing a lot of different angles of the body, so that the person seeing you will know exactly what you look like.
One picture enough is not enough on an online dating site. Since the advent of video online, people are getting more demanding to see more of potential suitors. People want to see others at various angles, close up and far away. They also want to be able to see their entire body.