Search Engines don't read graphics
The text content is the most important find for search engines. When the search engines crawl through all the sites they cling to words and phrases - this is commonly known as spidering. The words and sentences within your site in html mode are then added to the search engine database for future use.
When the customer then types words into the search part of the engine, ie what information they are looking for, the search engine delves into int's database and finds your site. If you have lots of graphic form and fancy downloads you could lose a lot of traffic to your site.