I think it depends upon where you live. I am from the South, and we say "spittin' image."
It is said both ways.
Some linguistic experts believe that the original saying was "spirit and image," which became corrupted to "spit and image" and then to "spittin' image."
1 In linguistics, the process of changing the meaning of a saying, through an alternate pronunciation, is called reanalysis. The saying, "splittin' image," refers to something far removed from "spirit and image." Reanalysis also explains the saying, "the birds and the bees," which was originally "the birds and the
beasts."