The idea of providing indexable content is to make it easy for search engines to crawl and understand your site.
Search engines can crawl images, however it’s really quite difficult for them to analyse and therefore are not ideal for communicating to search engines the topical relevance of your pages. For example, do the search engines really know that the image below is exactly ‘where Janine would like to be right now?’ unless I tell them by optimising the alt text and the file name when uploading the image.
Images alone are not enough to earn a web page top rankings for relevant queries although you should always ensure that the filename, title and alt attribute are all optimised accordingly.
According to Eric Enge: “While image identification technology continues to advance, processing power limitations will likely keep the search engines from broadly applying this type of analysis to web search in the near future.
Four ways you can use Google to search by an image
Drag & Drop
Drag and drop an image from the web or your computer into the search box on images.google.com
Upload an image
On images.google.com, click the camera icon, then select “upload an image” Select the image you want to use to start your search.
Copy and paste the URL for an image
If you’ve found an image on the web that you’re curious about, right click the image to copy the URL. On images.google.com, click the camera icon, and “paste image URL”
Right-click an image on the web
To search by image even faster, why don’t you download the Chrome extension or the Firefox extension. With the extension installed, simply right-click an image on the web to search Google with that image.
Bing introduces “Bing Visual Search”
I read today from Aaron Bradley at SEO Skeptic about the really impressive work being done by Bing right now. Their new Visual Search allows users to draw a box around an item in an image, and then Bing searches for it and returns images related to the selection. Check out The Bing Team’s post on Searching with Bing Visual Search