Image via WikipediaThe modern Christmas elf appeared as early as 1856 when Louisa May Alcott completed, but never published a book entitled "Christmas Elves". The elves can also be seen in engravings from 1873 in Godey's Lady's Book, showing them surrounding Santa while at work. I tried to find an image of this book, but could not. There were a lot of other images of her books, but not this particular one. I am always worried about copyright also. You get a lot of "images may be subject to copyright". Additional recognition was given in Edward Eggleston's 1876 work "The House of Santa Claus, a Christmas Fairy Show for Sunday Schools".
The image of the elves in the workshop was popularised by Godey's Lady's Book, with a front cover illustration for its 1873 Christmas Issue showing Santa surrounded by toys and elves with the caption "Here we have an idea of the preparations that are made to supply the young folks with toys at Christmas time". During this time Godey's was immensely influential to the birth of Christmas traditions, having shown the first widely circulated picture of a modern Christmas tree on the front cover of its 1850 Christmas issue.
St. Nicholas as an elf:
In Clement Clarke Moore's 1823 poem A Visit from St. Nicholas (more commonly known today as Twas the Night Before Christmas), Santa Claus himself is described in line 45 as "He was chubby and plump, a right jolly old elf".