Options to remedy sagging skin on buttocks from extreme weight loss? (photos)

Hello dear!
Thanks for the pictures and provided information as well.
Based on your photos, you are a good candidate for butt implants and fat transfer for contouring, to improve your buttocks, and I think you'll get very good results. 
l recommend you to make an in-person consultation with a board certified plastic surgeon for a proper answer and to talk about your goals and anything you need to know.
Good luck :)

Since your photos only show your buttocks, it is hard to say if a BBL is an option to fill out the loose skin envelope over your buttocks. We would need a large amount of fat to fill that loose skin out. You may want to consider a combination of fat transfer and posterior body lift, as a posterior body lift will flatten your buttocks even more than it is now.