Images are one of the most often overlooked areas of search engine optimization. It's easy to assume that an image can't be properly search engine optimized because it doesn't include text, but that's why there are special HTML tags and techniques specifically designed to properly classify and catalog images. Here's what you need to know about SEO image optimization.
The Alt Attribute
Usage: <img src="image.jpg" alt="Description of your image.">
Every image should have a concise, crisp description included within its alt tag. The alt tag is incredibly important; not only does it provide text for the search engine, but it is also used by screen readers for the blind -- and that's why an alt tag shouldn't be excessively long. A general SEO best practice is to include one or two descriptive keywords in alt text. These keywords can either be targeted towards the main keyword for your page or for the specific image that is being featured. Alt text is also what will be shown in the event that your image doesn't load, which can happen if there are network issues or server problems.
The Title Attribute
Used just like the alt tag, the title attribute also provides searchable text. The difference between alt and title is that the title is what will show up if someone holds their mouse over the image. Both tags are important and can be phrased differently to ensure the best SEO, though some simply place the same text in both slots. Either way, there are no negatives to including both an alt attribute and a title attribute.
The Image Caption
In addition to the attributes, SEO can also extend to image captions. Image captions (text under each image) will give the search engine more to work with and will also provide more context for your audience. Naturally an image caption functions just as any other text on your page; it should include some specific keywords that might be searched for when someone is looking for a relevant image. Image captions should be kept fairly short.
The Image Page
Many modern designs and content management systems link each image directly to an image page that includes a larger image, a description, a title, and other SEO text. An image page provides a searchable and indexed page in addition to the page that the image is featured on. This is an alternative to simply showing the image directly on the page.
SEO image optimization is deceptively simple; with just a few minutes of time, you can vastly improve the SEO of your website and search visibility of your non-text media.