From the currently known information concerning this lawsuit this appears to be a simple case. If a celebrity’s image is used without permission to sell a product, then the rights holder to that image is owed royalties for the sales and can block future sales.
However, it is not certain why the lingerie company thought it could use Monroe’s image. If they had reason to believe the estate gave them permission, then they might have a good defense to the lawsuit.
One thing can be learned from this: celebrities should make sure their likenesses are properly handled in their estate plans.