If your database is holding the field as a NVCHAR or similar, then it is indeed a string. In which case use
string s = (string)ds.Tables.Rows;
byte data = System.Text.Encoding.ASCII.GetBytes(s);
ms = new MemoryStream(data);
pictureBox1.Image = Image.FromStream(ms);
BTW: try not to use "magic numbers" for your code - it makes it hard to read and understand when you come back to maintain it.
are ok - ish - but
is not as readable as