How do I change a hero's walkabout and battle graphic set, whether the hero is in the party or not, and regardless of what slot the hero is in, using plotscripting?

The commands set hero picture and set hero palette can be used to change the walkabout and battle sprites of any hero in the party, whether active or not. To find out what the number of a certain hero is, you'll need to use find hero

Go into the Hero and Walkabout sprite menus and note down any picture and palette numbers you want to use.

Make sure to include mygame.hsi at the top of your script file (see HSI). Then write a script like the following, changing the example hero name and numbers to the real ones:

script, Bob's suit of armor, begin
  variable (hero)
  hero := find hero (hero:Bob)
  #change Bob's walkabout graphic
  set hero picture (hero, 17, outside battle)
  set hero palette (hero, 10, outside battle)
  #change Bob's battle graphic
  set hero picture (hero, 4, inside battle)
  set hero palette (hero, 10, inside battle)
end