Have you ever thinked about this

[sanjeeb]

I have used image on the header of pages in my website. now i want to know that can i use different kewords with the description of image in alt tag for each page of my site.

[Web Footed Newbie]

The alt tags in your header graphic should reflect what the graphic is, that is best practices. Although, you may use keywords that relate to the particular page in the alt tag, just don't overdue it (3-5 words maximum) and don't repeat keywords. ;)
WFN