Doctors in Dallas that does butt injections? (Photo)

Unless you mean fat transfer (aka BBL), i would not recommend going to anyone for injections of any foreign substance for buttock augmentation. Please read on:I have unfortunately had many patients come to me after having their high hopes for buttock augmentation crushed by measly Sculptra results. Critical points of awareness are: 1) Sculptra is temporary, it will last 1.5 - 2 years at most, 2) if done correctly dermal fillers like Sculptra are cost prohibitive for 99% of potential buttock augmentation patients. In order to create a significant/noticeable result with a dermal filler, at least 200 mls of product must be injected into each buttock cheek for a total of 400 mls. At $1000 per ml, this equates to a minimum price of $400,000.00. Thus if your doctor is recommending to put in ONLY 20 or 40 mls of sculpture you will pay $20,000-$40,000 and get an insignificant miniscule result!, 3) Do not let anybody tell you that the product is so special that it's going to magically grow over the following months or years...this just doesnt happen to any noticeable degree!, 4) Sculptra and other dermal fillers, when injected in large quantities, have a relatively high infection rate, guaranteed tendency to migrate away from the original area they were injected into, and almost always stimulate a lot of inflammation with a subsequent and disastrous amount of scar tissue/hardening, and 5) the one very serious complication that can rarely (< 1%) happen is "embolism" in which some of the filler gets into the blood stream and travels up into the lungs, heart, and/or brain causing serious problems, including death. 

Hope I at least saved you some money and maybe even your life. 

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.

The only type of buttock injection is fat grafting, otherwise known as Brazilian Butt Lift (BBL). This involves harvesting fat from around the body (lateral/medial thighs, abdomen, back and flanks, etc.) and transplanting it to the buttock. 
It is important to spend the time finding a quality surgeon educated and board certified in plastic surgery, active in the plastic surgery community, participation in peer reviews, over 10 years experience, and great reviews from previous patients.