When creating emails, it's important to note that you'll likely have a strong mix of both desktop and mobile viewers of your email. With this in mind, it's good to create responsive design templates for your emails. If you don't have the time or the ability to create a responsive design template, but still want to make sure that your images fit within the bounds of the email reader, here's a few tips for your email creation.
1) Use the following style parameter within your img tag to embed your images within table cells.
<img style="height:auto !important; width:100% !important;" />
2) Ensure that all your CSS styles, especially those related to tables and images, are written inline, and not in a <head> tag.
3) Keep your template format simple, and consider using only one column.