Image
Image is how a player is seen by others, as an indicator of the playing style that he is likely to use. Someone who folds a lot has a tight image, and someone who plays many hands has a loose image.
The simple explanation
Other players watch what you do and form an opinion. If you have shown only strong hands, you have a tight image, and people will believe your bets.
A closer look
A player's image may not match the way he really plays. Good players use this: with a tight image you can bluff more, and with a loose image you can get paid with strong hands. Your image changes with every hand that is shown, so think about what the others have seen. Also watch the images of your opponents, since they influence how others play against them.
A practical example
A player has folded for an hour, then raises. The others believe him and fold, because his image is tight.
Full explanation
Image is how a player is seen by others, as an indicator of the playing style that he is likely to use. Someone who folds a lot has a tight image, and someone who plays many hands has a loose image.
Image is how others see your style.
Tight means you play few hands, loose means many.
It may differ from your real play.
Use your image to bluff or to get paid.