The alt tag is the description for an image…
This tags exists since google cant “read” the image.
That is why we describe the image for googe / bing / etc… the alt tag should no be longer then 3-5 words max (3 is better). The title tag is not a must unless you wish to show additional information on mouse over.
Here is some additional information about the alt tag at google’s blog:
Hope this helps to answer your query.