New data from the dwarven caravan just in:
I got lucky and ropes weren't "in season" this year, so unless the images had special value, list price and selling price were identical.
Dwarfs
dislike: depictions of plants (larch, kobold bulb, palm), adjustment -16x quality
like:
- depictions of people and animals (forgotten beast, bat, deity), +20x quality
- minor historic events - foundation of the fortress, creation of masterworks, +20x quality
- a fellow dwarf admiring a preferred item, symbol of the civilisation(x), +20x quality
- major historic events - an elf slaying a cyclops, election of the expedition leader, +40x quality
- a fellow dwarf being terrified of despised vermin, +40x quality
The latter may actually be "creature" images, with the double modifier just being due to _two_ "creature" subjects present in the image (dwarf + vermin, elf + cyclops, exp. leader + appointment committee). Although i find the idea amusing that seeing a fellow dwarf cringe in front of jumping spiders is a sight dwarfs (and humans) will pay extra for.
they pay normal for images showing inanimate objects, be they geometric shapes, gemstone patterns or tradeable goods. Probably cheese, as well.
Since material modifiers enter into the calculation (as i said, everything was done in plant fibre, which has 2x value), this means that despised images are rated at 1/5 normal value, liked images at 2x normal value; and "double-feature" preferred images possibly at 4x normal value. These ratings are applied to the value of the decoration only, the value of the base item is unaffected.
There are a few special cases, obviously, when multiple very different things are featured in an image - e.g. a dwarf showing off a masterwork item - humans might pay 4x, while elves might give a bonus for depicting a living thing but at the same time a penalty for materialism (they pay less for any pictures featuring a tradeable item). I didn't have the opportunity to test this yet.
Elves don't seem to care much about non-image decorations, i only had a small sample and everything sold at list price. Haven't checked at all with humans so far.
Dwarfs like "menaces with spikes" and "encircled with bands of" decorations, those appear to give 50% value over list price. They dislike "hanging rings" (but happily slap them on their wares, so with a good eye, you can make some bargains on stuff they import); they're rated at 1/5 nominal value.
(x) the civ symbol might be valuable to the dwarfs only because it depicts a pond turtle man.
By the way: an elf slaying a cyclops is one of the most prominent historical events (seems i've settled in the back of nowhere again), and all three trading civs love that picture.
The biggest discrepancy between "basic value" (fortress-internal value rating) and trader price was a barrel the dwarfs brought along - it bears a fine image in black pyrope, netting a basic value of 620☼. But the dwarfs only want 140 to part with it, because it's an image of a larch.
PS: Combinations of preferred and disliked images just multiply their adjustments with each other - x0,2 for disliked, x2 for preferred for 0,4 total. Tested with elves, showing them an image of a dwarf showing off a masterwork item.
Elves only offered +50% increased buying price for an image of a kobold bulb, while trees enjoyed the full +100% markup. The same lowered markup was seen with an elf-brought image of a blade weed. Seems that it's a lowered benefit (still a significantly higher price than listed value!) for crops?