模块中的一些代码不懂
以下这些代码分别是什么意思？Public Const HTCAPTION = 2
Public Const WM_NCLBUTTONDOWN = &HA1
Public Declare Function ReleaseCapture Lib "user32" () As Long
Public Declare Function SendMessage Lib "user32" Alias "SendMessageA" (ByVal hwnd As Long, ByVal wMsg As Long, ByVal wParam As Long, lParam As Long) As Long
另，鼠标经过Image控件后，Image的Picture属性中的文件变换，当鼠标离开这个控件时，控件中的这个属性在次换回原先的图片，怎么做？
