Rihanna -- Hawaiian Ass Pic ... Dem Buns Are Da One

It's funny that Rihanna is the one in Hawaii ... because we're the ones enjoying the view.

RiRi is island side for the marriage of long time assistant Jennifer Rosales -- going down Monday -- but instead of taking in epic shots of perfect sunsets, she spent a lot of time this weekend snapping selfies of her perfect parts ... and we ain't complaining.

Front, back, side to side ... she definitely got all the angles ... but the trip isn't about her, right? It's about love and marriage and ... blah, blah, blah.