Search Engines, such as Google, Yahoo and Bing are constantly scouting the web looking for relevant information to index. They are interested in content in the form of words, not images.
This means that websites rich in graphics images, video clips, and the like may very well be too poor in textual content to give the search engines anything they can index.
This will result in search engines having difficulty finding your website. In the extreme, such sites can be virtually invisible to the search engines.