C# Add Border
static string[] AddBorder(string[] picture)
{
    var rows = new List<string> { new string('*', picture[0].Length) };
    rows.AddRange(picture);

    for (int i = 0; i < rows.Count; i++)
    {
        rows[i] = $"*{rows[i]}*";
    }

    rows.Add(new string('*', rows[0].Length));

    return rows.ToArray();
}

This builds a new grid with a * border around every side. It adds a full top and bottom row, then wraps each existing row from left and right.