A common issue encountered by our clients refers to distorted background images. This is caused mostly by using images which have different dimensions and aspect ratio.
The size of the image you upload should be the same that is recommended when you open the window to change the background image.
The height of the image should be equal or higher than the module height. The module height can be adjusted by adding or deleting content from it, or by resizing the heights available in the module if this option appears when hovering.