The best SEO (search engine optimisation) is having a website that is accessible, usable and useful. If you have good content, that can be accessed by people with a variety of web browsers, screenreaders etc., then that will attract more hits than anything else.
If you have images instead of text, search engines can't see it. That's bad SEO. Not to mention bad accessibility. (If you have alt text, this is not as much a problem.)
Moreover, if you have good content, people will link to you. This not only increases the number of people who visit from other sites, because search engines (particularly Google) take into account the number (and type) of in-bound links as well, your good content rises is more likely to be found by a search.