her response to the dm about finding some cheap artists says it all. i'm sure she didn't give her employees payment for making the posters in the past either.
Just because they're on payroll and being paid for their time doesn't mean you can instruct them to do anything you ask.
And besides, this is the U.S. Unless you’re in a union, responding to legitimate, legal and business-related tasks from your employer (that don’t exceed your ability to accomplish, e.g. not lifting 100lbs or whatever) with “not in my job description” will get you canned.
Most folks who do stuff like this just assume it's part of the job and carry on.