Search Engine Optimization and image have a relation. When you use image for text, then Search Engine cannot read what the image has said. This can be good for Adsense and bad for SEO. Adsense and other contextual advertising tool check the content inside an article and then place advertisements on the page.
Suppose you are talking about Search Engine Optimization on a flower-selling site. Then you may include some words that describe . In this case, Adsense may get confused and place adverts about flower. Which is not the right target for you website. Your website is all about SEO or Web development and you are getting SEO or Web development related visitors and adverts related to is not of use. You will get very less clickthrough, which will yield less revenue for your good article. In this case, Image is good for you in place of text. You can place small image in for few words. Like for words "", you can place this inside image. In this case, Adword tool cannot render the text and this will avoid adwords misunderstanding for contextual advertising (Adsense).
Another very good solution (programming) for the contextual advertising and poison words can be to use <noscript>poison words</noscript><script> document.write('poison words');</script>. This is very simple solution and it avoids the use of image. Image will not scale with text as easily as the text on the page when user resize text for his reading. You may also like to increase/decrease the font size in future. So, the above solution is very easy and non technical user can also learn to use. Just use the above code and place your words you do not want advertising bot to read for ads on your website.
Images are very bad for targeted keywords. If you are all about search engine optimization and you have created an image header where you placed all the target keyword then Search engine bot will miss all those keywords and you will not get ranking for all those keywords. In this case, where you want good-looking header and keywords in search result, you need to follow few rules.
First is to avoid Image for highly targeted keywords. Use text and use that text inside h1 tag or h2, h3 tag in order. H1 is the best. For very very highly targeted keywords you can use that text inside h1 even if the same word is inside image. It should be placed in a way that should not give impression to user that you are duplicating the content all over.
Second, is to use alt and/ or title attribute of image element. Alt/Title attribute can be read by search engine and can help to score some points for keywords. It will get less weightage then text on page but it is better than just images.
Third is to use this trick of SEO optimization for images. I do not like the solution but it can be useful for image on very important place or image at the top of the page. Research this yourself about its usefulness.
Images are not good for SEO but it is good for design purpose and for attracting user’s attention. It can provide elegance to Web design. It gives an article a voice. So, enjoy using images and write some text to describe those images so that Search engine optimization does not take a back seat.