初次发文，敬请包涵。
using System.Runtime.InteropServices;
窗口类添加以下成员或函数
private static int buttonIndex;
[DllImport("User32", CharSet = CharSet.Auto)]
public static extern IntPtr SendMessage(IntPtr hWnd, uint msg, uint wParam, ref Point lParam);
public const int TB_HITTEST = 1093;
//本例为四个按钮(注意：当添加属性style为separator的按钮后要作相应的变化)
//添加一个ImageList控件ImageList1(添加八个图片依次为0...7，顺序不能变，
//注意交互时图片0，1，2，3分别对应图片4，5，6，7)
//添加一个ToolBar控件ToolBar1(其ImageList属性为ImageList1，
//四个按钮的imageindex分别为0,1,2,3)

//工具栏MouseMove事件
private void toolBar1_MouseMove(object sender, System.Windows.Forms.MouseEventArgs e)
{
Point pt = new Point(e.X, e.Y);
IntPtr result =
SendMessage(this.toolBar1.Handle, TB_HITTEST, 0, ref pt);
buttonIndex = result.ToInt32();
const int lowBtnIndex = 0;
const int highBtnIndex =3;
if((buttonIndex >= lowBtnIndex ) && (buttonIndex <= highBtnIndex))
{
for(int u=0;u<4;u++)
{
if(u==buttonIndex)
this.toolBar1.Buttons[buttonIndex].ImageIndex=4+buttonIndex;
else
this.toolBar1.Buttons[u].ImageIndex=u;
}
}
else
{
for(int u=0;u<4;u++)
this.toolBar1.Buttons[u].ImageIndex=u;
}
}

//工具栏MouseLeave事件
private void toolBar1_MouseLeave(object sender, System.EventArgs e)
{
if((buttonIndex >= 0) && (buttonIndex <=3))
{
this.toolBar1.Buttons[buttonIndex].ImageIndex=buttonIndex;
}
buttonIndex=4;
}