Python Boto Script - Sorting based on date
The names will properly compare lexically even without converting them to datetimes, and you can just use the max function:

from boto.s3.connection import S3Connection
from boto.s3.key import Key

conn = S3Connection(XXX, YYY)
bucket = conn.get_bucket('bucketname')
rs = bucket.list("FamilyPhotos")
latest = max(rs, key=lambda k: k.name)
latest.get_contents_to_filename()