Thanks. According to the wiki, "picture" is just the object property of "ascii", and if I print it using ui.msg, I get a number that depends on the ascii property, so this seems correct. Is there another object property that holds the assembled sprite? I was thinking I might be able to register items with all the sprite and glow combinations I might need (I'm only planning on using pure red, green and blue glows) and then copy the assembled sprite property from an object of that item to the item whose sprite I want to change.
EDIT: So is there no way to change an item's description after it has been created? You didn't mention it, but the wiki says that the object property "desc" is the name of the object rather than the description, and doesn't list an object property for the description.