All of these Bilbos are by the Greg and Tim Hildebrandt, known when they painted together as the Brothers Hildebrandt. Greg and Tim were twins (Tim passed away recently), and when they worked together could produce amazing works at amazing speed working on the same canvas at the same time. When you see the work they did individually (and each is a great artist by himself) you can see what each brought to their collaborations, but I feel that when they worked together they created a special magic. Their images fleshed out my visions of Tolkien while I first started reading him; their 1977 Tolkien Calendar hung in my drama teacher's room that year. It was some years before I could see their work from 1976, but I made sure I got the 1978 calendar the next year.
The first Bilbo is from 1976. A detail from "Bilbo Meets Gandalf", posed for by Greg's son, Gregory Jr.
The second Bilbo is also from 1976, "Bilbo At Rivendell;" posed by Tim and Gregory Jr., but the face is of George Hildebrandt, the twins' father.
The third Bilbo is from 1977, "The Unexpected Party." I love this picture: it is perhaps my most favorite Tolkien illustration of all time. If I had the thousands of dollars necessary I would have bought the reproduction that used to be available at the SpiderwebArt Gallery, the Hildebrandts catalog site, and hung it up in my kitchen.
The fourth Bilbo is from "At the Grey Havens" (1978). Were there glasses in Middle-Earth? I don't remember them ever being mentioned, but several illustrators have shown Hobbits wearing them. Maybe it just seems to go so well with their general nature (like pumpkins, which people like to associate with Hobbits as well, though I don't remember them ever appearing).
The fifth Bilbo was produced quite recently, for the expanded edition of Greg and Tim Hildebrandt: The Tolkien Years. It's interesting to me that they kept in their vision of Bilbo's enormous sideburns (for consistency sake, I suppose). It just seemed natural back in the 70's, like Luke and Han's hairstyles in Star Wars, that now..., well, you know. The title of the whole picture was "Bilbo Meets Gollum." There's more Elvis than Elvish about it...