Is it right to say

I am going to the post office.
You have pictures of your nephew you mean to send to your sister. I can do that errand for you.
Can you please put these pictures of my nephew in the mail for me?

OR

You want your sister to send you pictures of your nephew.
Can you please send some pictures of my nephew to me?

Thank you dear Barbara. I meant your second suggestion, asking my sister to send me pictures of my nephew.

Of course, you could always use his name.

Maria, please send me some new pictures of Matthew.

Is it correct if I say: Please give Sara a kiss for me.

Yes. LIke the photos I would have put in the mail on your behalf you can ask someone to do something "for" you.

You won't be there to do it yourself, so they will do it for you.