1)首先将焦点放在您要截图的应用程序上.
link text

2)Win32 API可以帮助屏幕截图：

import win32gui
import win32ui 
hwnd = win32gui.FindWindow(None,windowname)
wDC = win32gui.GetWindowDC(hwnd)
dcObj=win32ui.CreateDCFromHandle(wDC)
cDC=dcObj.CreateCompatibleDC()
dataBitMap = win32ui.CreateBitmap()
dataBitMap.CreateCompatibleBitmap(dcObj,w,h)
cDC.SelectObject(dataBitMap)
cDC.BitBlt((0,0),(w,h),dcObj,(0,win32con.SRCCOPY)
dataBitMap.SaveBitmapFile(cDC,bmpfilenamename)
# Free Resources
dcObj.DeleteDC()
cDC.DeleteDC()
win32gui.ReleaseDC(hwnd,wDC)
win32gui.DeleteObject(dataBitMap.GetHandle())