Change default gamma theme image on homepage

HELLO FRIENDS
HOW TO CHANGE DEFAULT GAMMA THEME IMAGE ON HOMEPAGEON HEADER PLEASE?
MY WEBSITE IS aubainesenligne.com

I dont know the features of that theme but im pretty sure it must have that option integrated.
If you cant find it, please provide an admin details to login and i will try to find it for you.
If the function doesnt exist i will help you to replace it manually.

thank for your answer
do you want my login details here? or i can give it to yu by email

If you can message them here is fine. If you cant then send them to arcsalesltd@gmail.com

ok i will send you it by mail.

thank you so much. it is okay now