i assume you are trying to change the header logo. Give this a try.
put your logo image in the images folder of the theme you are trying to change.
in master.css find the section Master Header and change .header height to 200px.
Then change a.logo as follows:
that should do it.
take a copy of master.css before modifying just in case before modifying.
Good luck. Merry Christmas. Happy Holidays.