This is only a concern with gallery images / camera pictures. The wallpaper size required is a non-standard pixel count, so unless you've copied over a photo that's the exact right size, you're gonna have to crop.
TIP: When you see the green CROP box pop up, if you tap inside the box with two fingers, arrows will show up allowing you to expand and shrink the size of the box. Its a little finicky and takes some getting used to, but you'll be able to get your image as wallpaper with at least one full dimension.