All themes are required to be bundled with an accompanying screenshot image. This is to show an accurate representation of the theme to give users a quick preview when determining a theme to choose from.
The screenshot should be named preview.png, and should be placed at the top level directory of your theme. The image itself should be 1280px wide and 720px tall. The screenshot will be shown smaller, but the over-sized image allows for high-resolution viewing on HiDPI displays.