I can also reproduce this issue when access the shared file on a computer from Windows 10 IoT core.
The workaround for this issue is that you can setup a web server to share the file instead of using file sharing. For example, I setup the IIS on my computer and copy the signature.jpg to the root folder of IIS(C:\inetpub\wwwroot). Then we can use the code below download the file from IIS:
async void DownloadImageAndDisplay()
{
Uri fileUri = new Uri("http://{AddressOfPC}/signature.jpg");
IRandomAccessStreamReference thumbnail =
RandomAccessStreamReference.CreateFromUri(fileUri);
StorageFile imageFile=await StorageFile.CreateStreamedFileFromUriAsync("signature.jpg", fileUri, thumbnail);
BitmapImage bitmapImage = new BitmapImage();
Windows.Storage.Streams.RandomAccessStreamOverStream stream = (Windows.Storage.Streams.RandomAccessStreamOverStream)await imageFile.OpenAsync(FileAccessMode.Read);
bitmapImage.SetSource(stream);
image.Source = bitmapImage;
}