A few tips on how and when to use the alt attribute:
-Use the alt attribute for any image that is used as content.
-Use an empty alt atribute for any image that is decorative or not necessary for understanding the content of the page (alt="").
-Make sure the description of the image is useful. For example, if the image is your logo your alt should be your company name and not "logo"
The alt attribute is meant to help users using assitive techonology not miss any content, so make sure your text is helpful to anyone not seeing the image.