string name = "wallpaper.jpg";
var file = await StorageFile.GetFileFromPathAsync($"{ApplicationData.Current.LocalFolder.Path}\{name}");
var success = await SetWallpaperAsync(file);

async Task<bool> SetWallpaperAsync(StorageFile fileItem)
{
    bool success = false;
    if (UserProfilePersonalizationSettings.IsSupported())
    {
        UserProfilePersonalizationSettings profileSettings = UserProfilePersonalizationSettings.Current;
        success = await profileSettings.TrySetLockScreenImageAsync(fileItem);
    }
    return success;
}