Customising Simplex template

http://gogreene.co.uk/

I’m trying to add pictures into the header of the them simplex, but so far can’t get it to work. Somebody told me I might need to turn on my edit capibility in wordpress, but not sure where that is??

This is what I’ve tried so far:
Added the image in
Wp-content/themes/simplex/images/
I’ve then gone into
Wp-content/themes/simplex/css
I’ve tried to change the code from
#header {
Background: #28293
}

To
#header {
Background: #28293 url(images/Belsize park.jpg) no repeat
}

This wouldn’t save the changes

I was told to change this code in Appearance/editor/Stylesheet (style.css)
background: #**** url(‘images/****.jpg’) no repeat

But that code deosn’t exist there.

Any help would be greatly appreaciated.