This article covers the Legacy editor. Lessons built in the New editor work differently. To add an image in the New editor, see Adding images, tables and code to a lesson. New to the difference between the two? Start with The new lesson experience.
Adding an image
Open the lesson from My Library. The Legacy editor shows the lesson as a list of blocks, with the Outline down the left.
Click into the block you want the image in, then use the image button in the formatting toolbar and choose your file.
Click Save when you are done.