how can i add a button (link) across the top

I need to add a button so my users can return to the home web site
eg either after the Home button or before the LogOut button

where/what would I edit?

Admin CP > Display > Headers and Navigation > Nav links > Create a customized menu

This should be what you are looking for.

ok - found the page to add the button. but not a way to make an image for the button (to match the one in my style)
so I have to use photoshop?

Yes, you must make it or get someone to make it for you.

well - I did make one - I am pretty good at Photoshop
but STILL - HARD to match the existing font -
might have to make a WHOLE set to look uniform