The reference to Kant can be explained. There is no motion in the pictorial work. In at least one sense. The object, we think, just stays put. Yet we find motion there. The motion is not just in the depicting of a moving subject in the work. That is sort of obvious and in fact is not any motion at all. The sort of motion in the horses. We notice that they are not moving, in part maybe because we think they should be moving. Kant's lesson, Dufrenne says, is this: "movement in the subject precedes movement in the object."(278)

We might have expected such a lesson from Kant.  I am not sure exactly where Dufrenne is finding this, but in Kant we see:  “Motion of an objecgt in space does not belong to a pure science, and consequently not to geometry.  For the fact that something is movable cannot be known a priori, but only through experience.  Motion, however, considered as the describing of a space, is a pure act of the successive synthesis of the manifold in outer intuition in general by means of the productive imagination, and belongs not only to geometry, but even to transcendental philosophy."(Critique of Pure Reason, Kemp Smith B155 note)

Motion is brought to the object by the subject.  It is one more thing that the viewer brings to the work of art.  At least in the case of the pictorial art.  Even in the picture, we see first one thing, then another.  Our perception of the piece is “mediated by succession”.  So, the perceiver brings his attention to bear on the pictorial work, and in the nature of that attention, motion is provided to the work.  “It is by the movement of the look that the movement of the object appears."(278) So much is given by the spectator to the world at large, I suppose.  In everything I perceive my gave moves and so the object is endowed with motion, however static is might be.  In the work of art we are given something more.

Painting, say, can represent movement, but we want more than that.  We want the painting to be moving.  “representing movement and being movement are two quite different things for painting."(280) The painter can put movement into the work.  The lines on the canvass become something more than trajectories.  The artist embeds a meaning in the work to which the spectator may react.  The painter is moving the paint, and that is more or less obvious in the finished work.  There is, then motion left over from the creation that is meaningful in the finished piece.  As the brush strokes are often not carefully hidden or eliminated, but left to reveal what the artist was doing.  At the same time they reveal a motion present in the creation, the strokes also express the meaning (at least partly) of the work of art.

It is expressive, this movement in the work.  Yes, perhaps we need to admit, as Dufrenne does, that our speech here is metaphorical.  We might consider how musical expression works, and whether that is also metaphorical (reference here to Nelson Goodman, maybe).  In any case we may need to return to this consideration of the motion of the work of art, and how that plays to its expressive nature.  There may be a connection between the expressive in art and the intrinsic motion that exists in the work.