Ok, lets do some explanations here...
Mobile device in most cases is a portrait type device. If you want do display image that shows on entire screen this image needs to be also portrait. Now if you use landscape image - it is cropped on both sides just to cover entire screen. This mode is called cover mode.
Now if you would like to display entire landscape image on a portrait screen, you need to decide what to do - the only way to make the image fully seen is to sticks it to the top and you have 2/3 of screen empty (background color), or you can think about centring it vertically - then you have still background on top and bottom, or to stick it to the bottom.
Either case you stick with bad looking background here or there.
Same happens if you use portrait image with landscape device - then you will get background either on one of the sides, or partially on both.
Now you need to decide what to do next.