How to ask someone to take a photo?
There are many ways to ask this. Some examples:


您能给我们俩照一张相吗?
nín néng gĕi wǒ men liă zhào yì zhāng xiàng ma?
您能帮我们拍个照吗?
nín néng bāng wǒ men pāi ge zhào ma?
您可以给我们照张相吗？
nín kĕ yĭ gĕi wǒ men zhào zhāng xiàng ma? 
请问，您可以帮我们照张相吗？
qĭng wèn, nín kĕ yĭ bāng wǒ men zhào zhāng xiàng ma?
麻烦您给我们照张相
má fan nín gĕi wǒ men zhào zhāng xiàng 
麻烦你，帮我们照张相，好吗？
má fan nǐ， bāng wǒ men zhào zhāng xiàng，hǎo ma?


照张相 means to take a picture, so does 拍照（片）。 You can both use 能 and 可以. 帮（助） means to help. 我们俩 means the two of us. 给 is here a proposition.