The first Bilbo is by Michael Hague. His version of Bilbo looks very young, almost like a child; this impression is re-inforced by his teeth. Bilbo at the time of The Hobbit is fifty, and although some latitude must be given to the fact that a hobbit's life-span is slightly longer and they age a little slower, Bilbo should look more mature than he does here.
The second Bilbo is by Pauline Baynes, from Bilbo's Last Song. Baynes of course is famous for illustrating the Narnia Chronicles, but before that she had illustrated Farmer Giles of Ham, and went on to produce pictures for Smith of Wootton Major, The Adventures of Tom Bombadil, and covers for the Trilogy, so it is safe to say she had Tolkien's imprimatur. Her Bilbo has a nice balance of age and stoutness, so one can easily imagine him at ease at home or going on an adventure.
The third Bilbo is by Ted Nasmith, and is from a detail of Mr. Baggins at Beorn's hall. This picture begins to beg another question: do hobbits go bald, or get balding? Hobbit hair is always described as thick and curly, and Bilbo's hair is given as brown, although a lot of artists given him a sort of russet-red coloring. I seem to recall reading somewhere that Hobbits don't get bald, or was that Dwarves? More research is indicated.
The fourth Bilbo is by Alan Lee, perhaps the best all-around illustrator Tolkien has ever had, and certainly the most influential after his design work on the LOTR movies. His Bilbo is definitely middle-aged, and definitely balding (in fact he looks a little like Phil Collins to me), and his general proportions are good.
The fifth Bilbo is from a sketch by John Howe, who produced calendar work and book covers for Tolkien as well as being co-designer on the movies with Alan Lee. Although he has made some paintings of scenes from The Hobbit, Bilbo basically appears as a head shot in most of them, and they all look different. This is the only full-figure representation I could find. Bilbo here is on the younger side, and thinner, too; although I suppose it could be argued that Bilbo might lose some weight while travelling rough in the Wild.