How to retrieve the current user ID

I'm building a photo album. So, I want a photo to belong to a certain user. How can I get User ID?

var com = new SqlCommand("Insert into Photos (Photo, DatePublished, UserId)" +
    "values (@Photo, @DatePublished, @UserId)", con);
com.Parameters.AddWithValue("@Photo", pic);
com.Parameters.AddWithValue("@DatePublished", DateTime.Today);
com.Parameters.AddWithValue("@UserId", //what should be written here?);
con.Open();
com.ExecuteNonQuery();
lblStatus.Text = "Image Uploaded Sucessfully";

Or can you point out some other way to do this kind of belonging? Thank you.