list.sort accepts optional key function. Each item is passed to the function, and the return value of the function is used to compare items instead of the original values.
>>> my_list= ['image101.jpg', 'image2.jpg', 'image1.jpg']
>>> my_list.sort(key=lambda x: int(''.join(filter(str.isdigit, x))))
>>> my_list
['image1.jpg', 'image2.jpg', 'image101.jpg']
filter, str.isdigit were used to extract numbers:
>>> ''.join(filter(str.isdigit, 'image101.jpg'))
'101'
>>> int(''.join(filter(str.isdigit, 'image101.jpg')))
101
''.join(..) is not required in Python 2.x
