Select the course you'd like to edit.
Click 'Edit Course' to start editing course contents.
Once you've done that you'll arrive at the course creation page which looks like this:
Open your lesson text for editing.
Put your cursor to a place where you'd like to insert your image into.
Click on the link icon.
Enter the image link into the input, then click 'Save'.
You'll see the inserted image.
Click 'Save' to permanently store your changes.