The Bonsai in the picture is a 3.5 mt (10 feet) Ficus retusa linn that's over 1000 years old. It is also known as the Crespi Ficus, because it is exhibited at the entrance of the Crespi Bonsai Museum in Milan, Italy. The owner of this museum, Luigi Crespi, fought for decades for this unique Bonsai tree, and finally got ownership in 1986.
This tree was carefully crafted by the Chinese masters, and once it got to Italy it was properly taken care of by a man called Shotaro Kawahara, a Bonsai master from Japan who carefully pruned the tree to maintain its shape.
Though there were negotiations between China and Italy about the ownership of this Ficus Bonsai tree, Italy managed to keep its prized Bonsai beauty!
Yes. The bonsai is an artificial form for a plant. If planted on soil and let be, whatever plant it is, it will grow naturally into its full natural potential.
Lots of bonsai are plants which would normally become trees, notably small leaf maple, cypress, spruce and junipers. That said "tree" is loosely defined which I didn't know until today so I guess it's a matter of perspective but it seems to be mostly height based.
I'd say 2m or more with descernable bark and an elongated stem (trunk) it's a tree.