The best thing any performer wishing to be booked at any venue, is to have a Myspace or Youtube website.
I find a video or 2 of an actual performance tells me more than any CD can. Failing that a varied selection of songs or tunes on the Myspace or Youtube website to give the organiser an idea of the versatilty of the performer. A good example of what I mean is this website A very helpfull website from my point of view
If a performer sounds good, then I will wait to see them locally, live. If I enjoy that, I will invariably try to book. However, just becuase I like somebody, I still think about if the act will be suitable for the audience.
Once booked, all the performer needs to do is have a couple of very good quality photo's in action, that I can pick up by e-mail or from the website to put on the posters.
We do our own posters and promote the performer in a way that we think will attract an audience. What a performer thinks is important for promotion may not be how the organiser sees it.
A poster with just a photo on it, does not sell the performer, unless that person is well known in the area.

I hope that helps to see how I see things, but of course other people will have different viewpoints.