Use the alt attribute to give images a description, this is useful for getting indexed in the Google image index. Also, this is the text displayed if an image cannot be displayed because of a text browser or screen reader. Below is an example. <img src="http://www.a.com/a.jpg" [b]alt="Picture of an A"[/b]> Code (markup):
Yep, people try to make it out more than it is. All the alt attribute is used for is to display something if the image can't be, like with a text browser or screen reader. Google does use this alternate text to rank images in Google Images.