In order to add an image to a slide, just go to "Insert" in the upper settings menu of the editor, then Media and select Photo. In the pop-up window that shows up, you can search for keywords to find images related the topics you need.
On the left side of this browse window, you can also select to Upload your own pictures or even browse for Icons and Gifs.
In the Design editor, you can manually drag and drop the image in any desired position as well as to resize it by just extending or shrinking it. Clicking on the image, you'll find its editing options including its Caption, Crop/Style and Position.