With all sympathy for players who would like to play DF: the movie, I don't think that ordering a particular picture is that different from ordering a particular piece of furniture made from a particular material, or placing it in a specific place. Or ordering dwarves to become a full-time pump operator, whatever his preferences are. Not to mention digging out rooms of a specific shape on a specific place... and I'm sure a significant part of the playerbase would consider it a nightmare, should they dream about their dwarves digging of their own volition in the fortress. And it's just about the same thing: uncontrolled dwarves.
The distinction between functionality and creativity is a false one: sometimes the medium is the message, and the choice of material, decoration type or image might be used to honour, commemorate or mark (eg. uniforms) someone or something.
I do expect the subject of images, and particular types of decorations to have an effect on dwarves. If they do, they're functional. If they don't, it might as well say: "This boot is decorated with a random image".
Sometimes a player comes up with a crazy idea that cannot possibly be foreseen. If someone wants to send out an expedition with the specific goal to carve a sky-high obsidian obelisk, covered from top to bottom with engravings of the Carp, why shouldn't he?
I do think there should be significant risk be associated with forcing dwarves to do stuff they don't want to do. Ordering Urist Short-of-temper to engrave roaches, which he hates, should be enough to make him tantrum. Cog Dewbeard could go for suicide, while Lorbam the Restless will probably emigrate. (Also, since it should be a rarely-used feature, the normal, at-the-dwarf's-discretion engraving will be standard, and specific engravings will be hidden somewhere in the interface.) Offsetting this will require the player to play close attention to the dwarves' personal likes and dislikes, making their personality play a greater role, not a lesser.