Keywords in Image Tag Alt and Title Attributes
Search engines do index content found in image alt and title attributes—and placing keywords within the image tag can have a small positive impact on rankings. These attributes were designed to make images accessible to the vision impaired and to people who surf the web with image loading turned off in their browsers—and that's how these attributes should be used.
Content placed in image alt and title attributes should be descriptive and concise. Imagine a blind person is viewing your site using a screen reader. You should be asking yourself; What is the most effective way to describe that image to them? That's what you should include in the alt and title attributes, while still making sure the description contains one or more of your targeted keywords.
This is particularly important if the image is also a link. That's because alt attribute functions as the anchor text for that link. Link anchor text is a critical part of search engine ranking, so be sure your image links make smart use of the alt attribute.
Although the title attribute is less important than the alt attribute, it can still have a very small positive effect on rankings. The title attribute is used to add descriptive text to your html tags. And, even though most tags don't need descriptive text, it can still be an effective way to add some additional search-engine-friendly information to your links and images.
From web standards, accessibility, and usability specialist 456 Berea Street
One good use of the title attribute is to add descriptive text to links, especially if the link text itself doesn't clearly describe the link’s destination. This way you can help visitors know where the link will take them, possibly saving them from loading a page only to find out it wasn't anything they're interested in. Another potential use is to provide additional information for an image, like maybe a date or other information that is likely not essential.
It's important not to stuff these attributes full of repeated keywords. While we have seen sites achieve high rankings this way, it's a form of easily detected spam. Sooner or later sites that engage in this practice are detected and penalized. So, use your keywords in a way that appears natural to users (both sighted and vision-impaired) and you'll always be fine. Unnaturally forcing an excessive number of keywords into your pages can result in a ranking penalty.
If it makes sense to do so, images can also be named using your keywords (i.e. keyword.gif). Combined with alt and title attributes, this can also help these images to rank in a search engine's image search.
In the BIG picture, alt and title attributes are not critical ranking factors—but they do play an important role for image link anchor text and, used judiciously, are useful for placing keywords on your webpages.
Also note that if the image is purely decorative, such as a bullet or spacer, then it's a good idea to include these tags but to leave them empty. This is done by placing a blank space between the quotation marks; as in alt=" ". Bear in mind however that this practice has no affect on ranking, it just makes your site more usable to the vision-impaired and conforms better to HTML validation guidelines.
