The distortion itself is the Zapable software trying to make your image work on the screen it is displayed on.
There's two possible fixes for background images (and images in general) that are distorted.
If it's a background image you may have used one image for both portrait and landscape mode. This causes issues when landscape mode stretches your portrait dimension image.
If you can upload a differently sized picture.
It's also possible that you just need to resize the picture you uploaded (if it's a gallery picture for instance).