You can use the camera on your tablet (if present) or select a photo from anywhere on your computer.
When you click on the “Image” button you launch the Image Picker. In the Image Picker, if you click on the camera icon in the bottom left, it should identify your camera and allow you to capture images with the shutter button. If you have two cameras there should be a button to allow you to select which camera you want to use.
To select an image from your computer, click on the “Other” tab on the right. Then click “Change…” You will see a dialogue that allows you to select any directory on your computer.