You've got two methods:

1. Click on the "attach image" button, and enter a value into the height and/or width fields. It'll lock the proportions, so you only need to enter one value. You must enter the url of the image first.

2. Use the source, and follow this format for the img tag:
   

---

"you are terrifying
and strange and beautiful
something not everyone knows how to love." - Warsan Shire