[Header] How to I modify the header for a them?

Hello, dear wordpress admin:

I read several articles in support forum, I do found the place to put the code.
However, I can not get the image location right!!!!

For example, two situations:
1. I uploaded the img to /public_html folder
2. i uploaded the img to /public_html/wp-includes/images folder

How to link the url to the image in such two cases?

Please help!

Thank you