MainPage.xaml

<Page
    x:Class="UWPTest.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:UWPTest"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d">

    <StackPanel Background="{ThemeResource ApplicationPageBackgroundThemeBrush}" Orientation="Vertical">
        <Button Content="Browse.." Click="Button_Click"/>
        <Image x:Name="Img" VerticalAlignment="Stretch" HorizontalAlignment="Stretch"/>
    </StackPanel>
</Page>

MainPage.xaml.cs

using System;
using Windows.Storage.Pickers;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Media.Imaging;

// The Blank Page item template is documented at https://go.microsoft.com/fwlink/?LinkId=402352&clcid=0x409

namespace UWPTest {
    /// <summary>
    /// An empty page that can be used on its own or navigated to within a Frame.
    /// </summary>
    public sealed partial class MainPage : Page
    {
        public MainPage()
        {
            this.InitializeComponent();
        }

        private async void Button_Click(object sender, RoutedEventArgs e) {
            var picker = new FileOpenPicker() {
                ViewMode = PickerViewMode.Thumbnail,
                SuggestedStartLocation = PickerLocationId.PicturesLibrary
            };
            picker.FileTypeFilter.Add(".png");
            picker.FileTypeFilter.Add(".jpg");
            var file = await picker.PickSingleFileAsync();
            var image = new BitmapImage();
            await image.SetSourceAsync(await file.OpenReadAsync());
            Img.Source = image;
        }
    }
}