The image of a player is what others think of his style. They could believe him to be passive or aggressive, to be a bluffer, or one that only bets if he has a good hand, and so on. The image has a direct impact on the actions taken by the other players, and on how best to provoke a certain reaction from them.
The image becomes increasingly important as the level of play increases. The aim of building up an image is to lead the opposition into incorrect beliefs and moves. Hence, a player with a solid, conservative image can bluff more often since his opponents will believe him to have a good hand, whereas a player who is believed to be wild will have an easier time getting his good hands to pay off.