If there is an image on the web page, there should be an alt tag to describe the image.
Alt Tags are used to provide a text based descriptions for images on a web page. To add text to the alt tags – enter the text between the parenthesis
< img src="images.jpg" width="50" height="50" alt="This is part of optimization" >
There is a secondary aspect of the alt tag – it is used to provide information for the image for the sight impaired as a verbal description that gets translated into sound.
Although this too has been a point of abuse by many web site owners, the importance has diminished over time with this feature, but it is still a good practice and will also help with W3C compliance.