The "Fill" and "Contain" settings, within the "Image" tab on the right, are a great way to control how your images are displayed within the Image container.
Fill-- centers the image and fills the entire Image container. *This may crop parts of the image.
Contain-- preserves the original image size within the fields without cropping. *You may see empty space making up remainder of the image area.*
Below these options, if you have the Image set to "Fill" and you would like a different percentage of the Image visible, you can change the percentage, getting as close to what you're shooting for visually, when dealign with constrained dimensions for your Image container.
Background-- for a background Image, it is important to remember that the editor is 1200 pixels wide, while most computer screens are much wider. The Background Image will stretch to be the width of the screen. The height of the Canvas containing the Background Image, however, will remain the same height (unless you have set your Canvas to "Window Height" in the "Canvas" tab on the right).
Even if it appears that the Image fits well in the Showit app, change can be anticipated as the image height won't increase along with the width. Increasing the height of the Canvas can be the solution to your problem. Just make it taller or shorter, as you compare to the Published Page to see that The Background Image fits the best for your design!
Here's a great article on adjusting canvas height.