如果是因为画布背景色填充引起的，把画布的背景色填充为透明色，若果通过裁剪的方式，图片很难保证是原图等比例缩放

///生成缩略原图 
tImage = new System.Drawing.Bitmap(oWidth, oHeight);
gx = System.Drawing.Graphics.FromImage(tImage);
gx.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.High; //设置高质量插值法 
gx.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;//设置高质量,低速度呈现平滑程度 
gx.Clear(System.Drawing.Color.Transparent); //清空画布并以透明背景色填充 
gx.DrawImage(oImage, new System.Drawing.Rectangle(0, 0, oWidth, oHeight), new System.Drawing.Rectangle(0, 0, oWidth, oHeight), System.Drawing.GraphicsUnit.Pixel);