Who drew [the Oathbringer endpapers]? Are they in-world art?
These are in world paintings done by the Oilsworn, one of the people Shallan studied when practicing her art. The actual paintings were done by the Oilsworn's real-world counterpart, Dan Dos Santos, who did the cover of Warbreaker.
There are two more pieces in the back, done by someone else, which are also in-world art pieces. They're all part of a larger theme, and are equally gorgeous.
I assume these are paintings of heralds, then?
These two [in the front of the book] are Ishar and Ash. The back two are Jezrien and Vedel.
Ash is so much more...shiny than I expected, but I guess that's Lightweaving for you. I love the space background too.
Remember, these are in-world artifacts. So this is how someone painted her from their imagination, based on lore. These are Rosharan versions of the paintings of the prophets along the top of the Sistine Chapel.