처음 써보는 글이라 이 탭이 맞는지 잘 모르겠네요.

PrintWindow를 이용해 해당 창의 이미지를 가져오는 예제입니다.

[DllImport("user32.dll")]
public static extern bool PrintWindow(IntPtr hWnd, IntPtr hdcBlt, int nFlags);

[StructLayout(LayoutKind.Sequential)]
        public struct RECT
        {
            private int _Left;
            private int _Top;
            private int _Right;
            private int _Bottom;

            public RECT(RECT Rectangle) : this(Rectangle.Left, Rectangle.Top, Rectangle.Right, Rectangle.Bottom)
            {
            }
            public RECT(int Left, int Top, int Right, int Bottom)
            {
                _Left = Left;
                _Top = Top;
                _Right = Right;
                _Bottom = Bottom;
            }

            public int X
            {
                get { return _Left; }
                set { _Left = value; }
            }
            public int Y
            {
                get { return _Top; }
                set { _Top = value; }
            }
            public int Left
            {
                get { return _Left; }
                set { _Left = value; }
            }
            public int Top
            {
                get { return _Top; }
                set { _Top = value; }
            }
            public int Right
            {
                get { return _Right; }
                set { _Right = value; }
            }
            public int Bottom
            {
                get { return _Bottom; }
                set { _Bottom = value; }
            }
            public int Height
            {
                get { return _Bottom - _Top; }
                set { _Bottom = value + _Top; }
            }
            public int Width
            {
                get { return _Right - _Left; }
                set { _Right = value + _Left; }
            }
            public Point Location
            {
                get { return new Point(Left, Top); }
                set
                {
                    _Left = value.X;
                    _Top = value.Y;
                }
            }
            public Size Size
            {
                get { return new Size(Width, Height); }
                set
                {
                    _Right = value.Width + _Left;
                    _Bottom = value.Height + _Top;
                }
            }

            public static implicit operator Rectangle(RECT Rectangle)
            {
                return new Rectangle(Rectangle.Left, Rectangle.Top, Rectangle.Width, Rectangle.Height);
            }
            public static implicit operator RECT(Rectangle Rectangle)
            {
                return new RECT(Rectangle.Left, Rectangle.Top, Rectangle.Right, Rectangle.Bottom);
            }
            public static bool operator ==(RECT Rectangle1, RECT Rectangle2)
            {
                return Rectangle1.Equals(Rectangle2);
            }
            public static bool operator !=(RECT Rectangle1, RECT Rectangle2)
            {
                return !Rectangle1.Equals(Rectangle2);
            }

            public override string ToString()
            {
                return "{Left: " + _Left + "; " + "Top: " + _Top + "; Right: " + _Right + "; Bottom: " + _Bottom + "}";
            }

            public override int GetHashCode()
            {
                return ToString().GetHashCode();
            }

            public bool Equals(RECT Rectangle)
            {
                return Rectangle.Left == _Left && Rectangle.Top == _Top && Rectangle.Right == _Right && Rectangle.Bottom == _Bottom;
            }

            public override bool Equals(object Object)
            {
                if (Object is RECT)
                {
                    return Equals((RECT)Object);
                }
                else if (Object is Rectangle)
                {
                    return Equals(new RECT((Rectangle)Object));
                }

                return false;
            }
        }

        public static Bitmap PrintWindow(string procName)
        {
            RECT rc;
            var proc = Process.GetProcessesByName(procName)[0];
            GetWindowRect(proc.MainWindowHandle, out rc);

            Bitmap bmp = new Bitmap(rc.Width, rc.Height, PixelFormat.Format32bppArgb);
            Graphics gfxBmp = Graphics.FromImage(bmp);
            IntPtr hdcBitmap = gfxBmp.GetHdc();

            PrintWindow(proc.MainWindowHandle, hdcBitmap, 0);

            gfxBmp.ReleaseHdc(hdcBitmap);
            gfxBmp.Dispose();

            return bmp;
        }

소스코드를 여기에 붙여넣으려니 들여쓰기가 엉망이 됐군요

사용 방법은 PrintWindow("프로세스 이름"); 형식 입니다. 물론 반환 형식은 Bitmap 입니다.