Background image only shows the most left part.

Is it possible for you to provide a link to your site?

Yes ofcourse,

This is just a website i use to make concepts since i never got to make it to a real website.

Thank you for answering so fast.

You will need to adjust the CSS for background-size and background-position. Something like:

header#masthead {
   background-position: 50%;
   background-size: 100% 330px;
}

Your image does not seem particularly well suited to fit the header area so you may want to crop that too.

I am sorry i answered so late, i had a very busy period so didnt had any time to work on the website..
Do you know how i would change the bottom backgrounds too? Seeing as its divided into different parts and only the header changed :/
It kind of works though i guess.

Thanks in advance!