I often get asked for help with cropped and or stretched images in the Divi Theme's Slider and Header modules. The problem with the cropping is usually caused by having "parallax" enabled on the full-width header. Parallax gives the effect of the header background image scrolling at a different speed to the rest of the page. To do this it is necessarily to progressively reveal the top/bottom of the background image as the user scrolls, to mimic the effect of perspective. As a result, Divi needs to initially hide the top and bottom of the image from view. And to achieve this, it often needs to stretch the background image to make it the right size for this overlap. Note that this isn't something that needs fixed – it's just how parallax works.
To prevent this from happening, there are basically two choices – you can either disable parallax for the module, or you can attempt to edit the image in a program such as photoshop to make the image display better. If choosing the latter, you probably want to increase the height of the image while keeping the width the same. While you can add blank space when increasing the image height, note that this may become visible when the user scrolls, so it is better to extend the image with a color which blends nicely with the image, or crop a taller section from the original source image.