1) Go the the File Manager (in the Tools area of the admin)
2) Click on SkinImages folder to make it the active directory
3) Click on the Upload Button
4) Select your image and click the Upload button.
5) Now go to the Skins page for your site.
6) In the Customize Skin tab, you should see an option for selecting a background image. Click the drop drown and select the image you uploaded.
7) Click Save Skin Settings at the bottom of the page.
8) Remember that Pressero caches certain aspects of your site, including skin information. Go to /clearcache on your site to force the cache to reset.