Search engines are unable to view graphics or distinguish text that might be contained within them. For this reason, most engines will read the content of the image ALT tags to determine the purpose of a graphic. By taking the time to craft relevant, yet keyword rich ALT tags for the images on your web site, you increase the keyword density of your site.
Although many search engines read and index the text contained within ALT tags, it’s important NOT to go overboard in using these tags as part of your SEO campaign. Most engines will not give this text any more weight than the text within the body of your site.

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