You have all the parts, all you need to do is loop through the byte array and convert each number to binary representation.
Keep in mind there is usually a max length on a text box that you might have to manually overcome (make it multi-line / increase the max length, etc.
public static byte[] ImageToBinary(string _path)
{
FileStream fS = new FileStream(_path, FileMode.Open, FileAccess.Read);
byte[] b = new byte[fS.Length];
fS.Read(b, 0, (int)fS.Length);
fS.Close();
return b;
}
public static void SetText(string _path) {
byte[] b = ImageToBinary(_path);
StringBuilder binary = new StringBuilder();
foreach (var bb in b) {
binary.Append(Convert.ToString(bb, 2));
}
textBox1.Text = binary.ToString();
}