Many people pick a personal trainer by appearances only. Remember this: each personal trainer has started out with his or her own body as a baseline and then figured out how to successfully maximize THEIR potential. That may be radically different from YOUR baseline, goals, and potential.
For example, a genetically gifted bodybuilder personal trainer will not have personally struggled to overcome the frustrations and setbacks of losing weight and keeping it off, or spending great efforts in the gym but only making small improvements and hard gains.
Your goal should be to find a certified personal trainer with decades of experience in fitness and someone who started out in a similar situation to your own (overweight or hardgainer, etc).