//放大
    if MImageBoolean = true then
    begin
      SetStretchBltMode(ImagePaint.Canvas.Handle,STRETCH_DELETESCANS);//防止图片严重损失
      StretchBlt(ImagePaint.Canvas.Handle, ImagePaint.Left, ImagePaint.Top, ImagePaint.Width * 2, ImagePaint.Height * 2, ImagePaint.Canvas.Handle, ImagePaint.Left, ImagePaint.Top, ImagePaint.Width , ImagePaint.Height, SRCCOPY);
      ImagePaint.Refresh;
    end
    else
    begin
       //还原图片大小
      SetStretchBltMode(ImagePaint.Canvas.Handle,STRETCH_DELETESCANS);//防止图片严重损失
      StretchBlt(ImagePaint.Canvas.Handle, ImagePaint.Left, ImagePaint.Top, ImagePaint.Width, ImagePaint.Height, ImagePaint.Canvas.Handle, ImagePaint.Left, ImagePaint.Top, ImagePaint.Width * 2, ImagePaint.Height* 2, SRCCOPY);
      ImagePaint.Refresh;
      ImagePaint.Repaint;
    end;
    MImageBoolean := not MImageBoolean;

按照这样写的话，当图片放大后，再把它缩小时，在画布上会显示放大的图片和缩小的图片（本来应该只显示缩小后的图片，结果放大后的那张图片也在画布上）。请教，该怎么处理？

不太懂  先清空画布呢

如果在缩小前，先清空画布的话，那么之前在画布上画的图形也就没有了，再缩小就什么都没有了。画布是空的