There is no magic "xray" tool in photoshop, just as there is no magic video cam infra red settings that makes regular fabric see through.
What happens is that some cotton clothing and some bathing suits are actually see through, but use clever pigmentation to "distract" the eye from seeing what is underneath. In other words, such techniques would only work in situations where the person is either using a tight fitting cotton shirt that is actually see through or one of those "tan through" bathing suits.
You already "see" everything you want to see, it is just a matter of getting the colors right to make your eye ignore the pigmentation on the clothes.
In the case of the Zelwegger pic, as you can see it is not as tight as the shirt in the example, and on top of that there is some texture there. I.e. no way to see through. At best the nipple will become a little more salient, but that is because you can already see some of its contours on the pic.
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