OpenFilePicker.cs

// Create an OpenFilePicker starting in the pictures library, viewing thumbnails
var filePicker = new FileOpenPicker();
filePicker.SuggestedStartLocation = PickerLocationId.PicturesLibrary;
filePicker.ViewMode = PickerViewMode.Thumbnail;

// Filter to include a sample subset of file types
filePicker.FileTypeFilter.Clear();
filePicker.FileTypeFilter.Add(".bmp");
filePicker.FileTypeFilter.Add(".png");
filePicker.FileTypeFilter.Add(".jpeg");
filePicker.FileTypeFilter.Add(".jpg");
// Open a stream for the selected file
StorageFile file = await filePicker.PickSingleFileAsync();

// Ensure a file was selected
if (file == null)
{
    // Get a stream of the file – I’ll use this for uploading
    var stream = await file.OpenStreamForReadAsync();
}