Microstock, like most things, disproves the old saying, "Build a better mousetrap and the world will beat a path to your door." (Ralph Waldo Emerson?). If you want to make a sale, you need to get the image in front of people's eyeballs, and to do that, you need to find as many relevant keywords as possible. The more searches that hit your keywords, the more that your image comes up in front of designers. (Note that it is important not to "spamdex" your image in an attempt to get your image viewed using keywords that don't match the picture.