This question has been answered by BobRay. See the first response.
<img src="http://somesite.com/path/to/image/file" alt="Something" height="42" width="42">
This may help: https://sendgrid.com/blog/embedding-images-emails-facts/.Thanks!
The short answer is that the best method is to put the image somewhere it can be accessed by URL (e.g. CDN, or your assets directory) and do something like this in the email:
<img src="http://somesite.com/path/to/image/file" alt="Something" height="42" width="42">