WELCOME BACK!!! With a Shot of Rihanna's BUM In a THONG

Rihanna couldn't find a better way to celebrate her new pair of thigh-high boots so she took a photo of herself wearing them ... and a g-string.

RiRi tweeted 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.

Btw Nice Bum! *winks*