I performed a small test to see how image links pass keyword relevancy to pages. I wanted to see if the search engines would rank a page merely based on the keywords used within the image link. I first researched five made up words and made sure that no search results showed for each word. I then created five images with just the word in the image. I wanted to see if the search engines are starting to use optical recognition for images. I then created five new pages for each link. Each page had readable text, but no mention of the keyword within the text, file name, or meta tags. I placed the image links on the home page of an aged domain.
Within one week Google has indexed two of my five pages. Yahoo and Bing have not indexed any of them. What I found is that placing the words within the image itself had no impact what so ever in ranking the pages for the specific keyword. I also used the keyword as the file name for the image. This had no impact on rankings. I also tested the use of the ALT tag. Those links that had the ALT tag with the specific keyword in it had their associated page indexed for that keyword.
This test just shows that your image links with an associated ALT tag can pass keyword relevancy to your pages. I am going to wait and see if Yahoo or Bing will index any of the pages and write up a follow up.