i have a fresh magento 2 working. using Luma as my template. I want to add a costume image on the header that goes across the template, behind the logo. firebug directs me to .page-header pub/static/frontend/Magento/luma/en_US/css/styles-l.css - I realize this is static location. I am not suppose to edit it. where should i start? I just want edit the default luma theme I don't want add another costume theme. since i am using the github the templates are stored in frontend/Magento

I have a fresh Magento2 working and I am using Luma as my template.

I want to add a costume image on the header that goes across the template, behind the logo.

Firebug directs me to:

page-header pub/static/frontend/Magento/luma/en_US/css/styles-l.css

I realize this is static location, so, I am not suppose to edit it. 

where shouldI start? I just want edit the default luma theme and I don't want add another costume theme. Since I am using the github the templates are stored in frontend/Magento