绘制线的代码如下：
draw = ImageDraw.Draw(image_frame)
draw.polygon([(left, top), (left+width-1, top), (left+width-1, top+height-1), (left, top+height-1)], outline=(255, 0, 0))
保存为图片时会发现，线只有一个像素，怎么办？这个时候，移动一个像素，再绘制一次：
left += 1
top += 1
draw.polygon([(left, top), (left+width-1, top), (left+width-1, top+height-1), (left, top+height-1)], outline=(255, 0, 0))
如果觉得还是不够粗，再移动像素绘制．
