To insert an image, find the Image widget by expanding the "Widgets" menu on the left.
Drag the image widget to where you would like your image in your template.
Select the placeholder image by clicking it, and then click the "Choose Image" button in the right widget properties panel.
From the presented modal, upload or choose your image.
And voila! Your image has now been inserted into your template. You can use the properties panel to further adjust your image's settings.