Excuse me, Can I post your photo on facebook in mandarin
Normally, when you ask for permission, you can just ask it straight in a question form. In your case, you wanted to ask &quot;Can I post your photo on Facebook?&quot; would become &quot;我可以在Facebook上发布你的照片吗？&quot;

To be polite and show humbleness, adding a simple 請問 (may I ask) is enough -- &quot;請問我可以在Facebook上发布你的照片吗？&quot; (may I ask, if I can post your photo on Facebook?)

To show more humbleness, add 不好意思 or 对不起 is appropriate. You are apologizing for intruding that person's space and taking up that person's time; for even request that person to pay attention to your request.
&quot;不好意思 請問 我可以在Facebook上发布你的照片吗？&quot; (excuse me, may I ask, if I can post your photo on Facebook?)