Imagination is actually turning out to be a central part of Kant's version of how the mind works, although in addition to its normal usage Kant calls other things products of the imagination. There are two ways reason and imagination, as far as I've read, have been presented as working together. First off, reasoning in a sense depends on imagination for its operation because it's the imaginative faculty that takes sense impressions and welds them into concepts, concepts that reason then uses in order to make judgments and extract knowledge. Think of it this way: when you're thinking about a problem, or an issue, you're not directly dealing with sense impressions, not even if the things are right in front of you, but instead your mind is processing representations of whatever it is and manipulating those with logic in order to come to some decision. The faculty that takes the sense impressions and makes the concepts is imaginative because in a sense the conceptual representation is a calculated falsehood: it's false, because the concept of a chair, for instance, is something that goes beyond the individual sense impressions of the chair, but it's calculated because in making the sense impressions into concepts the mind puts in related knowledge and educated guesses gotten from memory and from previous experience.
In order to logically manipulate something, according to Kant, the 'something' has to be expressed in terms of a concept, as opposed to raw data. The manipulations themselves are what the categories deal with. The categories in Kant are integrated mental judgments about concepts that allow them to be used in particular logical processes. I see a chair, I have an idea of a chair, which is a concept, I apply the category of 'unity' to the chair and label it as 'one chair'. I touch the chair, get a sense impression of how it's cushion or lack thereof is, that's then turned into a concept, say 'hardness', which is a qualitative judgment. I can then relate the one concept to the other, by saying that "That one chair IS hard". Or, "That chair is one and hard, and therefore also not soft". I can look at it's material, which is a qualitative judgment that due to my imaginative faculty is then incorporated into my concept of the chair. It is wood. I know the concept of wood from previous experience, and that knowledge is added to my statement about the chair"That chair is one, is wood, and is hard". "Wood is hard, the chair is hard, the chair is wood". I can then go beyond strictly categorical thinking, and make a hypothetical statement using the categories to say "That chair is hard because it is wood".
Categorical judgments are analytic, that is to say that they're statements of fact rather than hypothetical statements, but the Categories themselves are a-priori synthetic qualities that concepts need to conform to in order for categorical judgments to be possible. Abstract logic in Kant's system is anchored in a-priori synthetic statements about the nature of concepts, their number, their qualities or lack thereof, as well as other more esoteric types of natures in the following way: in order to make quantitative judgments about something, about whether something is one, two, large, small, there has to be some basic idea about what quantity is. This idea, Kant argues, isn't gotten from generalization from experience but is hard wired into the brain, which is why it's a-priori. It's synthetic because we use the idea to generalize all the time about things that we perceive empirically, and we're usually right. It's not that much of a leap to say that evolution or nature over the course of millennia has deduced that saying that quantities exist out there in the world is a safe bet to make, and so has hardwired it into our heads to think that the world out there is possessed of quantity. Yet, our particular notion of what quantity is is something we impute to things. One coin or one rock makes sense, but what about the quantity of a messy desk? Is it a desk? Yes, but there are other things on it. We make things into wholes by our judgment when they could be further reduced into parts, and we do that in order to make useful sense of the world.
Likewise with quality. Despite the folks who followed Kant, who were called Idealists for a number of reasons, Kant himself did not belief that ideal archetypes existed. We see two things that are red. They're of different hues, but we still generalize that they're close enough to be called 'Red'. We've made a judgment and put that label on them, have in fact created the label 'Red' to correspond to a sort of sense experience that we have. Is there a basic 'Redness' behind things? Kant would most likely respond in the negative, and instead say that qualitative schemes of classification are created by our minds to make sense out of the world, but that our minds are also set up to make qualitative distinctions, even when we don't have much empirical data to go on.
*on edit: I've realized after the fact that I've made a serious error in representing Kant's position, but one that can be easily fixed.
Kant doesn't indicate a two step process of 1)conceptualization and 2) the application of the categories. Instead, he explicitly states that the categories are applied to sense data directly in the process of conceptualization. This means that when we take sense data and make the concept "chair", that the pure formation of the concept implies the category of quantity--a chair, and the category of quality--something that is "chair". Additionally, it implies the category of inherence, that something is "A chair", and also the category of existence, that "a chair is". Also implies "possibility", but I have my doubts about possibility as a category. In any case, these qualitative judgments are not separate from the concept and in fact describing them as happening in a separate step involves some redundant steps. For instance:
The fact that something is formed into 'a' concept involves the category of quantity. The fact that something is formed into 'a' concept that's different from other individual concepts involves the category of quality. But none of this disrupts the main point, which is that the faculty of the imagination is the thing that binds it all together.
Conceivably, the imagination could make products that violate the categories, and in fact during things like dreams, and times when we use the imagination on its own, in imagining a scenario, where reason comes second, categories are often violated and fantastic things often happen.