Why do Rebecca's breasts appear polished/shiny?
This picture shows Rebecca fighting in the Corrida Colosseum in  Dressrosa at the Mera Mera no Mi Tournament.

I the One Piece Wiki abpout Rebecca it's said:


  She is a fair-skinned, slender, curvaceous and very well endowed young girl


So we can assume that Rebecca has a very smooth skin. 
Since fighting is very sweat-inducing, we can say that her body is covered in sweat. 

Sweat + smooth skin = shiny skin in the right viewing angle