Weird reactions with header image everytime I change browser

Dear Friends

Being a "weekend" developper I am not always aware about all the tricks with CSS but basically I am about to release a new website for a humanitarian organisation

www.humanitarian-psy.org/new

On the homepage I have exactly what I need the logo and the gradient image, I added in ATO under style & edit Header Area

background: url(wp-content/themes/atahualpa/images/fractalbanner3.jpg);

Everytime I change browser to check the look & feel, my gradientimage (fractalbanner3.jpg) disappear. I have to go in ATO in the new browser and do a Save under style&edit Header Area.

I am looking since hours to try to understand cleaning my cache history. Any ideas, something wrong ?

Thanks a lot Fabrice