One of the simplest ways to make your Web site more accessible is to use an alt attribute in your image tags. The ALT attribute is designed to be an alternative text description for images. ALT text displays before the image is loaded (if it’s loaded at all) in the major browsers.
Why do you need it
The ALT tag adds a text description to an image on a Web page, and should be used for all images, graphical bullets, and graphical horizontal rules. The alt text within the ALT tag should let the user know what an image’s content and purpose are. ALT text give you more places to add SEO text that search engines won’t penalize you for. Use your search engine keyword where it fits with the description of the image. With image search, just as with web search, Google’s goal is to provide the best and most relevant search results to users. Adding ALT tags will increase the likelihood that your images will be returned in search results.
Here’s 3 best practices:
1. Tell as much as you can about the image
Don’t be so brief that the context is confused. Some people will only see the text in your alt tags, so if it’s too brief they might not understand what you’re trying to show them.
2. Provide good context for your image
The image and the content around it should be coherent. It will provide search engines with important information about the subject matter of your image and page.
3. Create a great user experience
Build traffic to your site using image content. It is recommended that when publishing images, you think carefully about creating the best user experience you can. You can do this by using good-quality photos and putting your images high up on the page.
How others have done it
Neil Patel described this topic here: http://www.quicksprout.com/2012/11/05/image-optimization-102-the-advance-guide-to-seo-for-images/
Please direct attention to images there. Every single one is nicely described in alt tag.