What if my body mass index is below 24? Does that mean I don't have enough fat for BBL? (Photo)

Hello dear!
Thanks for the question and provided information as well.
Based on your pictures, you are a great candidate for liposuction with BBL, and I think you'll get very good results.
Liposuction allows your plastic surgeon to transform your entire shape. You can improve your abdomen, back and flanks, and of course your buttock, to give you a nice heart shape.
In general, you need to be physically healthy and at a stable weight, have realistic expectations, be a non-smoker and your BMI needs to be 30 or less.
l recommend you to make an appointment with a board certified plastic surgeon to talk about your goals and anything you need to know.
Good luck :)

From your photos it seems as though you are a good candidate for a BBL. You should book an consultation with a board certified plastic surgeon who is experienced with BBL's for an examination and to further discuss your expected outcomes. 

Based on your photographs there seems to be sufficient fat for a BBL. Also sculpting of the hips and waist and flanks, greatly enhances the shape of the buttock. It is not all about just adding fat.  Fat can be harvested from wherever it is in excess, although the main areas are the abdomen, waist, flanks, and thighs.