I am interested in a bbl (skinny bbl) - would I need to gain weight first? (photo)

Thanks for the question. 
In this regard, I recommend you waist and back Liposculpture, getting better curves especially in the waist, improving your body contour.
Finally, I suggested take the fat we get and transfer to the buttocks and hips, to mold them and get better volume, contour and butt projection, is that we know as "Brazilian Butt Lift" (BBL).Kind regards

You would need to be seen in person by a board certified plastic surgeon to determine if lipo and BBL would make the difference you are seeking. The thinner patient requires an expert at liposuction to remove as much fat as possible while leaving the skin smooth. I have liposuctioned models and thin patients with good results.  There are a lot of women who can get nice results despite thinking that there is not enough fat.
Gaining weight is often counterproductive. If you gain weight that is fat, some of the fat will likely be associated with the buttocks.  In almost every case, I do not recommend weight gain prior to a BBL. I think it is better to maintain your stable weight.
Visit my website to view before and after photo galleries of my BBL patients and read patient reviews as you prepare to consult with a board certified PS via Skype or in person.