The picture that appears at the top of the website is put there by the theme. To change it,
Add the picture you want. (Be sure you think about how the words will appear–what parts of the picture they will cover.
To have a constant picture, just use one picture.
If you want multiple pictures to slide, then put more in the slider.
On the lower right corner of the page, the shortcode will appear. You can copy & paste the shortcode directly into any WordPress post or page.