Do search engines index ALT text descriptions for JPEG and other image files?
Yes, most search engines, including Google, look at your ALT tags when indexing your Web page. For many sites, particularly image-heavy sites, ALT text is one of the few elements available for the search engines to index. If your page has no text at all on it, it can still get a high search engine placement by using ALT text.
The overall impact of using ALT tags, in terms of search engine optimization, is low. As they have become abused (like meta tags ) by webmasters who fill alt tags with streams of keywords, the major search engines have lowered the importance of ALT tags in their algorithms. However, ALT tags are a required element for standards-based HTML coding. Every image must have an ALT tag, and each ALT tag must be properly implemented.
When adding ALT tags to your images, it is critical that you consider the visitors to your site, some of whom will be using browsers designed for people with visual, hearing, or other impairments. Others will view your site without downloading any of the images. The ALT tag enables you to convey your message no matter how your visitor views your site. The ALT tag must briefly describe the image, or if the image includes words, must repeat the words in the image, so that your visitors who cannot see your page will get the information conveyed by the images.
Every image on your site should have an ALT tag, even if the image is just a spacer. For spacer images, usealt=””. That will inform people using speech reading browsers that the image is to be ignored.
For more information on the correct usage of the image ALT tag, please visit Objects, Images, and Applets in HTML documents – 13.2 Including an image: the IMG element on the World Wide Web Consortium website.