Read the latest! Check out this more recent post from Boostability: What is alt image text and why should I use it?
Alt image attributes (also known as “alt tags” or “alt text”) are brief text statements that you add to an image on your website. The Alt attribute was developed for the visually impaired reader, who is using a screen reader. The visually impaired reader will hear the alt text in place of the image. A text browser such as Lynx will display the Alt Attributes text instead of the image. A graphical browser typically will display only the image, and will display the Alt Attributes text only if the user asks it to show the image’s properties. Many graphical browsers can be configured to show the alt text instead of the image.
By adding Alt Attributes, visitors with image-disabled browsers are able to know what the image represents. It is also meant to be used as an alternative text for when the image is unavailable to view. If an image is unavailable, a small icon is shown with the Alt Attributes Image Text like this:
Here’s an example of an <img> tag and the attributes that it should include:src=”/images/example.gif” alt=”Image Alt Text Goes Here” title=”Image Title Goes Here” />
The “src” attribute contains the source or location and file name of the image so that the correct image is shown.
Alt attributes are useful because they let the search engine robots know what the image represents. According to SEOMOZ‘s Search Engine Ranking Factors, keyword use in image alt text is the 13th most important on-site SEO factor, so be sure to incorporate your keywords into the image alt tags.
Remember that Alt Image Text’s function isn’t to describe what the image is, but to state the function of the image and what it’s purpose is. A good example of this is the home page button. You wouldn’t want to put “little house” as the Alt attribute, you’d want to put something like “Dallas Texas Computer Hardware Home Page”. It’s supposed to be an alternative for the image, usually stating its purpose. For example, an image of a STOP sign should not have alt text “an octagon with red background, black border”, but simply “STOP”. (Unless of course the purpose is actually to show what the warning symbol looks like.)
It’s highly recommended that you don’t stuff your image alt tags with too many keywords as this may trigger spam filters which can hurt your site’s ranking. It’s recommended that you use only one keyword per image. The World Wide Web Consortium (The official authority on Internet rules and standards) says that alt text is required on all images. “The generic rule for the content of the alt attribute is: use text that fulfills the same function as the image.”
It’s not necessary to put image alt attributes on background images since they are not essential to the informational content of the page.
Don’t be confused by the “title” attribute because it’s function is different from the “alt” attribute. Image title’s are mainly used to show a mouse-over text on images. According to searchenginejournal.com, the image title should “provide additional information and follow the rules of the regular title: it should be relevant, short, catchy and concise.”
The alt attribute is commonly referred to as the “alt tag” or “alt text”.