Update: with the right plug-in, you can often ensure that the images are automatically included in the RSS feed. An example of this: Featured Images in RSS for Mailchimp & Other Email
Our email designer automatically retrieves all the information that is in your RSS feed URL. This means that image elements will be displayed that are located in other elements in your feed, such as the description tag (<description>).
For example: <description><!CDATA[<img...
If you want to display a specific image in your email, you must first ensure that this image is set as its own element in your RSS feed.
For example:
<enclosure url="http://mysite.com/image.jpg" length="12345" type="image/jpeg"/>
or
<media:content url="http://mysite.com/image.jpg" type="image/jpeg" medium="image" />
Once your feed contains your image elements, you can add them to your email. You can then edit the block and under the 'Customize' tab, you will see the 'Content URL' or 'Enclosure URL' as available elements. You can then select these to add.