How can I make him send me pics? should I ask it polite or how?

hello everyone. I am texting with a guy almost 2months, but he doesn't still send me his pic with full of face, only whatever parts of his face mostly his chin, lips. and when i ask he says he can't send (because of his job for security) or suddenly changes topic. we talk on phone, he is kind, but i dont know how should i ask him? he always finds excuse, i dont know if i ask it in a calm way or should i insist till he sends? i dont want to say him if he doesn't send i will cut him off, by the way sorry for my English
How can I make him send me pics? should I ask it polite or how?
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