Comics, like film, portray something visually specific, so in that sense they're "micromanaging imagination", yes. Obviously novels and other print forms require you to imagine the appearance of the characters and settings, and comics generally don't...though there are slight exceptions: for instance, an extremely abstract drawing style could lead the audience to extrapolate what a character would look like in real life, as in Sin City for example. You're technically "spoon-fed" an image, but it's quite different from reality, so in that regard there's a leap of imagination required. In general, however, yes, you are being given something to look at and identify with the story.
But that's only one aspect
of imagination. You still need the imagination to bring the images to life, as has been mentioned, and to create a character's "performance" from a series of still images. In comics, the audience is the actor--they're given a physical form for the character, but the subtleties of how he or she moves, talks, fidgets, and expresses emotion are in the audience's hands.
Which brings us to the other major act of imagination: the other four senses, particularly sound. That's a bigger deal than you might imagine, especially when comics start playing around with lettering and sound effects. "Pogo" was the king of this, using lettering in elaborate ways to suggest character but which could never sync up with "reality", like the character "P.T. Bridgeport", a flamboyant salesman who talked in Circus posters:
The key to encouraging the imagination in comics rather than micromanaging it is to think abstract, to move away from reality.