Currently it looks like you are getting the title from this code.
Users users = (from x in myEnt.Schwarzes_Brett where (x.ID == id) && (x.Titel == titel) select x).FirstOrDefault();
The linq is getting only the First or the default, you need to select all of them, so I might make
List<users>
and then you could use linq to retrieve all of them to a list
List<users> users = (from x in myEnt.Schwarzes_Brett where (x.ID == id) && (x.Titel == titel) select x).ToList();
Then for each User in that list you could add the picture to your grid view