思路：
1、首先截图整个页面
2、定位截图的对应元素
3、获取该元素的坐标及大小
4、从大图截取小图
代码：
# coding:utf-8
from selenium import webdriver
from PIL import Image
driver = webdriver.Chrome()
driver.get('http://www.baidu.com/')
driver.save_screenshot('button.png')
element = driver.find_element_by_id("su")
print(element.location) # 打印元素坐标
print(element.size) # 打印元素大小
left = element.location['x']
top = element.location['y']
right = element.location['x'] + element.size['width']
bottom = element.location['y'] + element.size['height']
im = Image.open('button.png')
im = im.crop((left, top, right, bottom))
im.save('button.png')
