To think Ive played this game a lot and haven't done this yet....
Well, time to play through again.... Does the difficulty matter or can you do that on either mode?
Viewing post in Trial of the Demon Queen comments
I just checked, since I didn't remember, and past me decided that bonus art is only unlockable with normal difficulty.
That's not great, if I did it today I'd definitely make it available in easy mode as well (in fact that'll be the case with the sequel), but that's how it works in this game unfortunately :/