A friend of John or a friend of John's ?
When 's is required ?
Should we write "a friend of John" or "a friend of John's" ?
The 's is required in this case. Grammarians call this construction double genitive or double possessive. They say a double possessive is essential to avoid giving the wrong meaning when a word indicating ownership is placed after of.
The extra possessive is required because "a picture of Jane" means an image of Jane, whereas "a picture of Jane's" is a picture of any sort that happens to be owned by Jane.