The assets panel (former left Materials panel) provides some images and materials you can use in scenes. Use the dropdown control at the top of the panel to select the type of assets you need, then browse the assets in the list below.
Dragging and dropping
You may drag materials directly to the scene, or to the materials list at the right panel. You may also drag them to the disk and then back to the scene or to the right panel. You may have your own collection of materials, if the built-in one is not enough.
The same for images, you can drag them onto the shapes or to the scene background. Boxshot shows a popup menu when you drop an image to the scene, so you can choose how to apply the image.