Whether your site focuses largely on images or not, everyone should be providing as much information as possible for the images they use. If your site does focus solely on images, i.e. a wallpaper site, you’d be a fool not to provide any more information than the text on the page.
As far as search engines have advanced over the last decade, it still takes a human eye to be able to tell what content an image contains. However, many search engines are capable of determining how similar one image is to another. This is evident in Google Images if you do a search you might notice that there are no duplicate images – which makes sense, why show the same content twice? If anything, this highlights the need to use original images if you want some traffic from Google Images. Again, original content is king. Now, when Google spiders crawl your site to index new pages or updates, they have no idea what the images relate to or what is actually shown in the image unless they are given some information in text form, a language they do understand. Search engines will take the general topic of your page (page title, on page keywords, url keywords) and assume it is somewhat related to your image. Aside from this, you have a few chances to give them specific information for your images in the form of the image alternate text, file name and the title tag.
The main purpose of the alt text tag is self explanatory, it provides Google with alternate text it can use in place of the image. It is said that the alt text of an image used as a link carries the same weight as anchor text in a normal text link. Anchor text is a hugely important ranking factor and I’ve seen it improve search results dramatically so alt text definitely shouldn’t be overlooked.
The title tag determines what text shows when a visitor hovers over an image, it’s simply intended as something to help your visitors so putting some relevant info in there is another opportunity to describe your image and Google will pick up on this. Lastly, the file name of your image is again just another chance to throw a keyword or two into your page and will help you rank better.
Implementation is simple, be short and sweet with your text:
<img src="google-spider.gif" alt="Google Spiders" title="a Google Spider" />
With correct use of the above, you will find it is remarkably easy to rank well in Google Images. If your website is image based (wallpapers etc) then you should try optimizing all your pictures, Google Images could be a huge source of traffic for you.