Images can be added in the Outline and Design view of the editor. To add one, go to "Insert" in the upper settings menu, choose Media and then Photo.
A browse window will pop up for you to type in keywords and search among hundreds of images from the built-in gallery.
You can also upload your own images, by selecting the "Upload" input on the left side of this window, or even insert Icons and Gifs form the given galleries too.
Once you add the image, in the Design editor you can click on it to find its options: Caption, Crop/Style, and Position.
In order to resize the image, just manually extend it from its edges in the design editor. You can also move it around freely in order to reposition it.