public函数在timer中执行设置背景图片失败

如题，写的是一个判断光标位置的函数
问题在于他的backgroundimageChange事件会触发，
确认图片有效，且没有逻辑错误，
但是显示出的效果始终没有变化

以下为代码

public void icon_bgset(int Mouse_X,int Mouse_Y)
        {
            //取坐标
            int FrmX = locationX + deskTop.DeskTopLocation.X + 8;
            int FrmY = locationY + deskTop.DeskTopLocation.Y + 30;
            int FrmWidth = this.Width;
            int FrmHeight = this.Height;
            //判断鼠标位置
            if ((Mouse_X >= FrmX && Mouse_X <= FrmX + FrmWidth) && (Mouse_Y >= FrmY && Mouse_Y <= FrmY + FrmHeight))
            {
                deskTop.plans = 1;//光标在icon界面时候

            }
            else
            {
                deskTop.plans = 0;//离开icon的时候

            }


            switch (deskTop.plans)
            {

                case 1:
                if (this.BackgroundImage == null)
                {
                    //Image winBackimg = Image.FromFile(Application.StartupPath + "\\img0.jpg");
                    Bitmap spp = new Bitmap(Application.StartupPath + "\\BKGD.png");
                    this.BackgroundImage = spp;

                    DeskIcon.Icon_ISIN = true;
                }
                    break;
                case 0:
                if (this.BackgroundImage != null)
                {
                    this.BackgroundImage = null;
                    DeskIcon.Icon_ISIN = false;
                }
                    break;
            }
        }

https://blog.csdn.net/sinat_20265495/article/details/50766379