as long as we are able to perceive two or more things (ideas, objects, etc etc) we have the ability to imagine. e.g why are these two objects here? in asking the question, one has created with his imagination not only a question, but also a perceived number of objects, and a place in which they are.
so imagination could be described, in this instance (everything is only objectionably relative of course), as the creation of ones perception by a means of deducing what is known into that which is assumed.
when only one thing is known, only our own existence is comprehendible, as the first thing would have to be understanding of knowledge of oneself. however the duality of existence also concludes that infinite things are needed to imagine. so perhaps non existence is cause enough for imagination...
by ones own understanding of oneself, one has created the second thing, his own understanding..
but in saying that, concludes that one was able to imagine or create oneself from non existence, or could existence be without knowledge of itself?
also, if infinite things where needed to imagine, there would be no need for an imagination, so i guess, the less that is perceived, the more imagination one would need, or, to imagine more, one would need to perceive less (or perceive everything)