I will run my curser around looking for a way to get to the website. I will look for links in the text, or images in the body of the email that are links. If they aren't there then I'll check to see if the logo is a link or the header. Nope. What do I do then? I give up and don't visit the site and so do most people.
You have to make it easy for people to come to your site. You have to make it as easy as possible.
In the example below of Sarah's Sewing Center, Sarah could have used many links. She could have made text links to the classes schedule when she mentioned classes. She could link to the new Amy Butler fabrics on the website (an an image would have gone far there) and she could have linked again to the store website and the online store mentions all in the body of the email.
It's also a good idea to have a link to the website on the header or with a logo or something and a map link at the footer or a side bar would be great.
The point is, don't waste the time and energy you're putting into your email newsletters by not making it easy for people to visit your website. Each newsletter should generate a spike in site views and that can translate more easily into sales.