Welcome to Wikidot. This snippets site is not a general support forum, so in future you'd be best to post questions like this on the community forum at http://community.wikidot.com/forum:start where you'll get a quick response 24 hours a day.
But to answer your question, there are a couple of ways of adding a background image to all pages. One is to add it into the live template page(s) _template on the site. The other is by changing the CSS (custom stylesheet). With CSS you can change every element on your page and create your own theme so it's worth learning.
But it depends where you want the background image as to what the CSS rule is that you will need to set. We'll show you a couple of couple of examples.
On your mzansiforce site you have a page called admin:themes which you can access by entering that in your browser address bar (http://mzansiforce.wikidot.com/admin:themes).
First of all upload the image that you want to use to that page using the Files button at the bottom of the page.
Then edit that page and you'll see a a codeblock starting [[code type="css"]]. In that codeblock below the @import rule add the CSS rule for the background image:
background-position: center top;
An example of the result of that can be seen on my test site here where the whole background is an image.
But make sure in your site manager at admin:manage -:> appearance -> themes you set the theme to be an external theme /admin:themes/code/1
If you just want an image in the content part of every page then you can either add it to your _template page like this:
Or you could add it to the CSS in the same way as in the first example but using the #main-content rule instead of #container-wrap.
You might also want to read some of the how-tos about designing your site which you can find at http://community.wikidot.com/howto:_start. One which might help you adding your own CSS is http://community.wikidot.com/howto:flexible-css-putting-css-on-its-own-page.
Depending on the theme you are adding to you might need to make other changes to the CSS as well to get the result you want. In which come back to us on the community forum.