Question solved!

PIL.ImageGrab.grab(bbox=None, include_layered_windows=False, all_screens=True)

And it captures every screen.

I've faced this same issue. If I had to guess, I'd assume that ImageGrab is getting its coordinates from the SM_SCREEN flag, which gives the coordinates of the primary monitor only, rather than the SM_VIRTUALSCREEN, which gives the entire virtual screen (I haven't looked through the source though, so just a guess).

That said, it's easy enough to get around this by interacting directly with the Windows API, then converting its bitmap output into a more usable PIL object.

Some Code:

def _get_screen_buffer(self, bounds=None):
                # Grabs a DC to the entire virtual screen, but only copies to 
                # the bitmap the the rect defined by the user. 

                SM_XVIRTUALSCREEN = 76  # coordinates for the left side of the virtual screen. 
                SM_YVIRTUALSCREEN = 77  # coordinates for the right side of the virtual screen.  
                SM_CXVIRTUALSCREEN = 78 # width of the virtual screen
                SM_CYVIRTUALSCREEN = 79 # height of the virtual screen

                hDesktopWnd = windll.user32.GetDesktopWindow() #Entire virtual Screen

                left = windll.user32.GetSystemMetrics(SM_XVIRTUALSCREEN)
                top = windll.user32.GetSystemMetrics(SM_YVIRTUALSCREEN)
                width = windll.user32.GetSystemMetrics(SM_CXVIRTUALSCREEN)
                height = windll.user32.GetSystemMetrics(SM_CYVIRTUALSCREEN)

                if bounds:
                        left, top, right, bottom = bounds
                        width = right - left 
                        height = bottom - top
                
                hDesktopDC = windll.user32.GetWindowDC(hDesktopWnd)
                if not hDesktopDC: print 'GetDC Failed'; sys.exit()
                
                hCaptureDC = windll.gdi32.CreateCompatibleDC(hDesktopDC)
                if not hCaptureDC: print 'CreateCompatibleBitmap Failed'; sys.exit()

                hCaptureBitmap = windll.gdi32.CreateCompatibleBitmap(hDesktopDC, width, height)
                if not hCaptureBitmap: print 'CreateCompatibleBitmap Failed'; sys.exit()
                
                windll.gdi32.SelectObject(hCaptureDC, hCaptureBitmap)

                SRCCOPY = 0x00CC0020
                windll.gdi32.BitBlt(
                        hCaptureDC, 
                        0, 0, 
                        width, height, 
                        hDesktopDC, 
                        left, top, 
                        0x00CC0020
                )
                return hCaptureBitmap

        def _make_image_from_buffer(self, hCaptureBitmap):
                import Image
                bmp_info = BITMAPINFO()
                bmp_header = BITMAPFILEHEADER()
                hdc = windll.user32.GetDC(None)

                bmp_info.bmiHeader.biSize = sizeof(BITMAPINFOHEADER)

                DIB_RGB_COLORS = 0
                windll.gdi32.GetDIBits(hdc, 
                        hCaptureBitmap, 
                        0,0, 
                        None, byref(bmp_info), 
                        DIB_RGB_COLORS
                )

                bmp_info.bmiHeader.biSizeImage = bmp_info.bmiHeader.biWidth *abs(bmp_info.bmiHeader.biHeight) * (bmp_info.bmiHeader.biBitCount+7)/8;
                size = (bmp_info.bmiHeader.biWidth, bmp_info.bmiHeader.biHeight )
                print size
                pBuf = (c_char * bmp_info.bmiHeader.biSizeImage)()

                windll.gdi32.GetBitmapBits(hCaptureBitmap, bmp_info.bmiHeader.biSizeImage, pBuf)

                return Image.frombuffer('RGB', size, pBuf, 'raw', 'BGRX', 0, 1)

The first function gets a bitmap of the screen, and the second guy converts it to a PIL object.

If you don't want to figure out the coordinates of the other monitors yourself, I've got a small module called PyRobot which has functions for targeting specific monitors and whatnot. And it's pure Python, so no need to install PyWin32 :)

Unfortunately it isn't possible, due to the manner in which the PIL obtains the dimensions of the display device. When it obtains the Device Context, it does obtain one for all attached monitors:

screen = CreateDC("DISPLAY", NULL, NULL, NULL); 

(display.c, line 296, version 1.1.7)

However, to get the display dimensions, it uses this code:

width = GetDeviceCaps(screen, HORZRES);
height = GetDeviceCaps(screen, VERTRES);

(display.c, lines 299-300, version 1.1.7)

Which only returns the dimensions of the primary, active monitor. All subsequent operations are done with these width and height values, resulting in a final image that is only the size of the primary display.

In order to receive a screengrab of all attached monitors, those two lines would need to be replaced with something like:

width = GetSystemMetrics(SM_CXVIRTUALSCREEN);
height = GetSystemMetrics(SM_CYVIRTUALSCREEN);

After which you'd need to recompile PIL. This would provide you with the dimensions of the virtual screen, which is "... the bounding rectangle of all display monitors." [MSDN]

A more correct implementation would be using EnumDisplayMonitors to obtain device contexts for the individual monitors, along with altering ImageGrab.grab()'s interface (or adding a new function) to allow for the selection of a specific monitor, of whose device context would be used for the remaining operations.