Use the toolbar buttons at the topThis is the easiest way and the one you use primarily. Just click on the icon corresponding to the object you want to see appear in the center of the slide. You can then modify it according to your wishes.
Copy/Paste from Windows
You can also copy/paste texts or images from any other Windows application using the standard copy/paste functionality.
Similarly, you can select other objects in other applications (text, Excel charts, etc ...) and place it directly on the target slide.
When you select multiple images and you put them down on a slide, they are all added to the current slide.