1. From your dashboard Click on Library.
I find it useful to right-click on it and open it in a new window.
Makes it easier to go back and forth between the library and your editor.
2. In the Library, Click on "Add"
3. Click on "Choose file"
4. Select your file and enter a name in the field
5. Click on Save
6. In the library, find your file, and copy the URL
7. Now go to your Email menu
8. Click on Add Template
9. Inside the email editor, click on the image icon to Insert/edit image
10. Paste the copied URL into the field. You can adjust the size of the image numerically, or pull the corners of the image once it's placed inside your template.
11. Click on Save