Here is a Python 3 version using Selenium webdriver and Pillow. This program captures the screenshot of the whole page and crop the element based on its location. The element image will be available as image.png. Firefox supports saving element image directly using element.screenshot_as_png('image_name').
from selenium import webdriver
from PIL import Image
driver = webdriver.Chrome()
driver.get('https://www.google.co.in')
element = driver.find_element_by_id("lst-ib")
location = element.location
size = element.size
driver.save_screenshot("shot.png")
x = location['x']
y = location['y']
w = size['width']
h = size['height']
width = x + w
height = y + h
im = Image.open('shot.png')
im = im.crop((int(x), int(y), int(width), int(height)))
im.save('image.png')
Update
Now chrome also supports individual element screenshots. So you may directly capture the screenshot of the web element as given below.
from selenium import webdriver
driver = webdriver.Chrome()
driver.get('https://www.google.co.in')
image = driver.find_element_by_id("lst-ib").screenshot_as_png
# or
# element = driver.find_element_by_id("lst-ib")
# element.screenshot_as_png("image.png")