They aren't particularly exclusive. Skill allows artists to convey what they imagine: without it, all sorts of information is lost. An unskilled artist trying to convey an idea may succeed on a few levels, but I guarantee that the full idea, in all it's glory and precise emotional tonalities, will not come through.
Imagination is 100% necessary for art. If you can't visualize what you want to make, you just won't be able to make it. That said, absolutely everyone has some base level of imagination all ready to be amplified by skill, with the possible exception of some people with severe mental disabilities.
In any case, having a good imagination is no excuse not to build up your skill. If anything it should be more of a reason to practice, so that you can express your wonderful ideas fully and perfectly. Imagination x skill = art, but you can only properly control skill and imagination never equals zero.
OP, what sparked this question?