Ok got one step further:
This is what I have used and it seems to work:
.header-image {
position: fixed;
}
#access {
border-top: 1px solid #EAEAEA;
border-bottom: 1px solid #EAEAEA;
position:fixed;
}
BUT: now the header image suddenly automatically changes….
any idea how to leave the size of the header image untouched?
Thanks!