In math the kernel is the inverse image of a subset of the image of a some map, were the subset is equal to the identity element in the codomain. I'm certain these names derive from mathematical concepts as they are related significantly in various fields in mathematics. Considering Unix was derived in an academic environment it may be possible that it's use of these word's kernel and image are the same.
If you have a set which represents some level of information about the "complete" O.S., if that information also forms a group then you can define group homomorphism's on that set or basically maps to other sets having different sizes then the original set so long as they "respect" the orginal set's structure that made it a group. You can see it may be in one's favor to map the set to a smaller set or a subset of some set where the subset is smaller.
Image - The image of a group homomorphism and in general functions and maps, are just a subset of some set, who's elements actually get mapped to. The function may not map to every single element and those elements would not be included in the image.
Kernel - Basically just the elements from the original set that map to the image, but only map to the identity element in the image. Basically the elements that map to 0 like thing in the image.
If the image is smaller in size then the original set then we can see multiple items must map to one single element. So for example there may be multiple elements from the kernel that map to the image and we already know they all have to map to 0.
We can see that if we choose the original set to be finite sequences of binary or 1's and 0's and the codomain (set mapped to) to be also sequences of binary, then we can construct such things if and only if, a suitable group structure can be defined (this little bit in depth and unrelated to question asked).
So we see with complete certainty that "kernel" and "image" of an O.S. are completely defined and have mathematical meaning. Independent from perhaps other uses of the terms.