NOT okay under ANY circumstances. That's why companies like [her company] do
rights clearances for images of celebrities that they sell. Anything
being produced for commercial purposes (to sell rather than say
editorial/news purposes) must get permission from the celebrity's
management or estate. UNLESS the image is so old that it's considered
public domain and there's no estate.
Hope that helps!
At the same time, what are the chances that the poster would be seen? If
it's small, then maybe a risk worth taking?