It may seem a bit of a surprise, but the same techniques to improve search engine optimization are inherent in Accessibility features. Let me explain. Search engines crawl the web using a "bot", for example Google uses Googlebot and Bing uses Bingbot. The bot is responsible for two primary functions, the first is to find all the links on the page to add them to search fetch list. The second function is to index the words on the page. Bots do not have eyes nor do they have ears, so they do not see images or hear sounds, they can only "see" text.
By following some of the accessibility rules, you will be able to make the bot "see" things on a page that it would have missed without accessibility techniques. For example placing a few descriptive words on an image "ALT" attribute is meaningful and visible to bots while text embedded in images is not.
Although, "A picture is worth a thousand words", when it comes to SEO, a thousand words in any form is not necessarily a good thing. All major search engines rank pages based on the quality not the quantity of the content. A good splash of keywords is great, but if you overdo it your rank will drop. You don't need to avoid images, just keep your content out of them.
For more information about accessibility see: