We can make written notes, and we have paint, and we have a tooltip display for sketch images that are applied to objects. Why not let us have drawn notes as well as written ones? Add an illustration to a story, leave an illustrated card for a special event, make an illustrated flyer for the village notice board, etc. (And maybe also allow us to paint runestones too).
The image wouldn't be displayed in-game for parchment, only on right-click, the way writing already is only visible by right-clicking the parchment.
Alternatively allow us to "complete" sketches so that they can't be used for crafting, only for communication as-is.