The old saying of a picture is worth a thousand words is anything but true when it comes to search engines. For the time being, search engines cannot view an image and then know what keywords to associate them with. Instead, search engines rely on the webmaster to give an indication of what the image is about, via the alternative text feature.
In, select an image, and then press the Media Inspector button. In the Alt Image field, type in your Alternative Text for your image. Remember to be descriptive, while not keyword stuffing. Avoid making your alt text misleading in any way.
Remember, try and include the keywords or phrases you have been using throughout your site while also keeping the text short, to the point, and relevant to the actual photo. Do this for all images you deem necessary to having alternative text. Do your best to avoid putting too many words in the alt text.