Well, here are some nuances. I'm writing this as a personal style guide, not as an authoritative position on what is correct and what isn't.
She has nice legs = Ea are picioare frumoase.
This is clearly meant as a general statement/impression about a person, so "picioare" is used without any definite article; otherwise it would sound a bit weird to me.
She has clean legs = Ea are picioare curate. / Ea are picioarele curate.
When "picioare" is used, this alludes to the fact that she generally keeps her legs squeaky-clean. But when "picioarele" is used, it somehow hints that her legs are clean at this moment.