"Not one of my great photoshopping skills but I'm still on it, to get the hang in these things.

Involved was:
Imgur for hosting the images
Firefox Firebug: For inspecting elements, which were stored at the awesome imgur picture hosting side
Steampunk Mrs. Foster with the awesome MK23 from Killing Floor
and Twilight Sparkle from My Little Pony: Friendship is Magic.

How I changed the background image:
This is a simple project of saving a website locally (Save as > *.html) and editing it's components at will using element inspection.
The code for replacing the background images are 2 parts:

1: The background image in the center
<div style="background-image: url(http://i.imgur.com/xzKETFG.png);" class="profile_background_image_content ">
</div>

2: The background image on the visible left and right side
<div style="background-image: url( 'http://i.imgur.com/xzKETFG.png' );" class="no_header profile_page has_profile_background ">

You notice the imgur link in both codes? That's what came as a replacement."