The exact likeness.
One of the very first questions that was asked at the Phrasefinder bulletin board was about 'spitting image'. There have been numerous such queries there since and some ask if the term was originally 'splitting image', i.e. deriving from the two matching parts of a split plank of wood. That's a plausible idea. The mirror image matching of the grain of split wood has long been used in furniture and musical instruments for decorative effect. The technique is known as book-matching and the resulting pattern is called fiddleback - for obvious reasons. The theory has its adherents and dates back to at least 1939, when Dorothy Hartley included it in her book Made in England:
"Evenness and symmetry are got by pairing the two split halves of the same tree, or branch. (Hence the country saying: he's the ‘splitting image’ - an exact likeness.)"
As so often though, plausibility isn't the end of the story. The numerous forms of the term 'spitting image' - spit and image, spitten image, the dead spit of etc., appear not to derive from 'split' but from 'spit'.
Some commentators have suggested that 'spit' may be a corruption of 'spirit', but that appears to be fanciful and isn't backed up by any early examples of 'spirit and image'. The allusion is more likely to be to someone who is so similar to another as to appear to have been spat out of his mouth. That idea, if not the exact phrase, was in circulation by the end of the 17th century, when George Farquhar used it in his comic play Love and a bottle, 1689:
"Poor child! he's as like his own dadda as if he were spit out of his mouth."
No version of the phrase is especially old. The earliest reference is in Andrew Knapp and W. Baldwin's The Newgate Calendar, 1824–26:
"A daughter, ... the very spit of the old captain."
This pre-dates any 'splitting image' citation by a good hundred years, which tends to rule out the latter as the source. 'Spit' or 'dead spit', with the meaning of likeness, appears in print several times in the 19th century. Here 'dead' means precise or exact, as in dead ringer.
Other languages have their own versions of this phrase; for example, French - "C'est le portrait craché de son père" ("He's the spitting portrait of his father") and Norwegian - "som snytt ut av nesen paa" ("as blown out of the nose of"). These are difficult to date and may predate the English version or may derive from it.
Toward the end of the 19th century we find 'spit and image'. In 1895, an author called E. Castle published Lt. of Searthey, containing the line:
"She's like the poor lady that's dead and gone, the spit an' image she is."
Finally, we get to the first known use of 'spitting image' - in A. H. Rice's Mrs. Wiggs, 1901:
"He's jes' like his pa - the very spittin' image of him!"