When you add an image to a page on your website, you can also add an “alt” property to the image. Many people call this the “alt tag”. This alt-property explains, in plain text, what the image is a picture of.
This is great for accessibility, especially for visitors who use a screen reader or other assistive technology (see below). It’s also a good opportunity to slip some extra keywords into the page. A word of warning however – some unscrupulous SEO types will tell you to push lots of keywords into these alt-properties and over-stuff them.
What your users want is a clear, concise description of the image, and this is what the search engines will be looking for too. Don’t be tempted by the Dark Side.