"spitin' image" and "spit and image" are both corruptions of "spirit and image"

Sounds a bit too "clean" to be true to me, IP.

How about the meaning of "spit" as in a slender pointed rod for holding meat over a fire [M-W]?
Once upon a time this rod would have been wood rather than metal, in which case "spitt'n image" may have been akin to "chip off the old block". Are spit and splinter related??

Another alternative is taking "spit" as both something that is spat out, and something pointy that sticks out (spit of land). If someone is the "spit" of someone else they are their "issue" (from inside) and their "extrusion" (outside). You can't get much more closely related than that!