It's your money and personality bro, you are free to decide on what you want without anyone infringing on your privacy rights. Some people might be camera-shy while others might not just want it due to popularity problems and some ugly aftermaths.

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Are gambling platforms paying an extra money for the publicity?
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It depends on the arrangement and how the winner in question understands the situation.

Although it might be in the terms and conditions of the casino that they have the right to use a certain winner at their discretion for such a task. In that case, the winner will have no just but to oblige. But in a different case, the winner might charge them extra or do it for free, it all depends on the person in question.