ALT tags should contain image relevant keywords and be used on each image of a website. Using ALT tags site-wide will increase exposure to image search engines.
Hello kumarpyasa Alt tag is meant to be an alternative information source the both tags are primarily meant for visitors (though alt text seems more important for crawlers) so provide explicit information on an image to encourage them to view it or get them interested. Include your main keywords in both of them but keep them different. Keyword stuffing in "alt" and "title" is still keyword stuffing, so keep them relevant and meaningful...
ALT tag is used for identifying things like pictures with an appropriate text for those who can't view images
Alt tag used in image where u can use keywords in alt tag. Its help to generate traffic for your site. Google can't read image so by alt tag your site images can appear in google images search.
ALT text is used by screen readers to speak page content to a user - it should represent what the image is. BUt try and get your keyword in there as well.
I am agree with your comment, because crawler are not able to crawl images, so in images we can use alt tag and in alt tag we can use our keywords.
If you use images in your Web pages, its good practice to include Alt text for every image that a visitor to your site will see. ALT tags filled with keywords can also be used to boost your keyword frequency and help you achieve better rankings.
alt tag is very useful for image search. <img src="smiley.gif" alt="Smiley face" height="42" width="42" /> alt tag describe about your image.