I do not use them for marketing, nor do I think of them as a gimmick.
If I use ‘before’ and ‘after’, it is to illustrate something to my clients. I use them most often to show postural improvements, particularly since I take pictures as part of my initial assessment. My clients are very motivated by seeing how, for example, their forward rounded shoulders and incorrect head position is moving towards better alignment. Those pictures stay between the client and myself.
Of course I, realize that you are thinking of those famous weight loss successes, and I am happy for any trainer who has such a client. But even there, the credit for the success goes to the client, not the trainer. There are some clients who are in the right frame of mind to attack those extra pounds and who happen to latch onto just the right trainer for them. And that combination can bring the most rewarding result. But it is the synergy that produced it, not the trainer. For that reason, I would not use such pictures as marketing material even if my client gave me permission.