0x00.添加引用

using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Controls.Primitives;
using Windows.UI.Xaml.Input;
using Windows.Storage;
using System;
using Windows.System.UserProfile;
using Windows.ApplicationModel;
using System.Collections.ObjectModel;
using Windows.UI.Xaml.Media;

0x01.选择图片

FileOpenPicker openPicker = new FileOpenPicker();
openPicker.ViewMode = PickerViewMode.Thumbnail;
openPicker.SuggestedStartLocation = PickerLocationId.PicturesLibrary;
openPicker.FileTypeFilter.Add(".jpg");
openPicker.FileTypeFilter.Add(".jpeg");
openPicker.FileTypeFilter.Add(".png");

var file = await openPicker.PickSingleFileAsync();

0x02.复制所选图片并设定

if (!UserProfilePersonalizationSettings.IsSupported())
{
    MessageDialog md = new MessageDialog("Not Support");
    md.ShowAsync();
}
else
{
    StorageFolder base_folder = await Package.Current.InstalledLocation.GetFolderAsync("Assets");//选择App安装目录下Assets文件夹作为复制目录
    await file.CopyAsync(base_folder,file.Name, NameCollisionOption.ReplaceExisting);//将选择的图片复制到base_folder
    StorageFile copy_file = await StorageFile.GetFileFromApplicationUriAsync(new Uri("ms-appx:///Assets/" + file.Name));

    UserProfilePersonalizationSettings setting = UserProfilePersonalizationSettings.Current;
    await setting.TrySetWallpaperImageAsync(copy_file);//设置为壁纸,返回true成功,false失败
    await setting.TrySetLockScreenImageAsync(copy_file);//设置为锁屏,返回true成功,false失败
}

PS.如不将所选文件复制至App目录下会出现权限问题