Firstly, I’ll let you know what alt attributes are in case you haven’t heard of them before. Alt attributes show alternative text that is displayed if an image is missing or a tool tip if the user hovers their mouse over an image. Alt attributes are also important for visually impaired readers. You will see many images on websites that will have have alt attributes that just reflect the images file name.
e.g.
<img alt="DSC007.jpg" src="/images/DSC007.jpg" width="800" height="600" />
This bad for search engine optimisation. Your alt attributes should contain a small amount of data that gives a brief description of what can be seen in the image. It’s best to use images that are relevant to your SEO campaign so that you can use your keywords in the alt attributes so that Google has more keyword repetition and density. If the example I used above was a picture of a person wireframing while he was redesigning a website and you were targetting the terms “website redesign” then a good alt attribute would be something like this:
<img alt="A man in the process of a website redesign." src="/images/DSC007.jpg" width="800" height="600" />
Don’t stuff your keywords into your alt attributes!
Black hat search engine optimisation “experts” used to perform the practice of keyword stuffing both in content and in the alt attributes of images. They would often only put the keywords in all the alt attributes. This would sometimes get effective results for a short period of time but in the long run Google would often black list the site in it’s results. Make sure that this doesn’t occur during your SEO campaign! This practice is very unethical and is frowned upon in the SEO community.