My butt crack is short but my butt starts higher than the crack.

It is very difficult to determine the exact size and shape implant you will require to best match your ideal butt image without an examination by a board certified plastic surgeon. Not just any board certified plastic surgeon, but one with many years of frequently performing breast augmentation surgery including different approaches, techniques and implant choices. This is because several measurements not to mention your breast characteristics are needed to determine the optimal implant size to obtain your goals. Without knowing these dimensions it would be difficult to make this determination. For example, the existing base width of your buttock will determine, in many cases, the maximal volume per implant profile that you can accommodate. Liposuction of your lower back and fat transfer is also helpful as an adjunct to implants. Using “want to be” photos however are useful if simply provided to the surgeon.

It is hard to answer without photos and you should resubmit the question with photos. Possible that a fat transfer to your upper buttock would balance your short buttock and give you a more normal "crack". Resubmit or just see a plastic surgeon who does a lot of fat transfers or BBL. Good luck.

This is a really difficult question to answer without any photos. Nonetheless, a short buttock crack is often due to a pack of fullness in the upper half of the buttock cheeks necessary to create buttock cleavage (aka "butt crack"). If this is the case for you as well then augmenting the buttock cheeks with buttock implants is the best option for a permanent result. Glad to be of help. 

Hello dear!!
Thanks for the question and provided information as well.
Unfortunately, without pictures, it is very difficult to give you a proper answer. 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 :)