Rihanna -- Thongin' Out in Risque Instagram Pic

Rihanna got a new pair of thigh-high boots yesterday ... and to celebrate, she took a photo of herself wearing them ... and a g-string.

RiRi instagrammed the pic ... along with the following message: "Gotcha bitch tip toeing on my marble flo'!!! Sick ass custom Prada boots! Miuccia you RULE!!! Thank you."

No, Rihanna ... thank YOU.