The first picture of Bilbo, by Susan Greenwood Sweeton, is from a magazine ad for silk screen T-shirts that I saw pretty regularly in the 1970's; other characters offered were Gandalf and Smaug. This illustration comes down on the rotund side of interpreting the Hobbit's propensity for fat, which varies from obese to pudgy to pot-bellied to almost nonexistent.
The second picture is of the aged Bilbo at Rivendell, and is a detail from a large poster by Judy King Rieniets (this poster was later reproduced as the "centerfold" of one of the Tolkien calendars). Rieniets has a definite Celtic feel to her designs.
The third Bilbo is a detail of a picture by Tim Kirk, one of the first artists to produce a Tolkien calendar. It is from a famous and iconic scene, the Riddle Game with Gollum. Illustrators love this scenario. The different interpretations of Gollum would be an interesting study in itself.
The fourth picture is from a teacher's catalog for Cassette Classics, which sold packages for units on different subjects. As such, the style is geared more to appeal to children. Once more, it's from the Riddle Game. The only identification for the artist is the name "Berrett" in the corner. The date is uncertain, but probably the early 1980's.
The fifth picture is by Darrell K. Sweet, and is a detail from one of the Great Illustrator's Edition Tolkien calendars. I like Sweet's work; he went on to produce an entire calendar's worth himself and his pictures were chosen as covers for one of Ballantine Books runs of issues of Tolkien's works. His Bilbo seems very thin and lanky, and this is accented by his short vest.