To upload the image to your server:
Login to your WP dashboard
Click on Media from the Media menu widget
Click on the Add new link under library in the widget box or the Add New button on the page next to the page title
If using the flash uploader click Select Files/if using the browser uploader click Browse
Upload the file and copy the File URL
To replace the default background with the newly uploaded file
Click on Appearance
Click on Antisnews CSS Styling Tool
Click on Manage Options next to Page Body CSS Settings
Go to Page Background Image and replace the current URL with the URL to the background image you uploaded. If you copied the File URL as instructed when you uploaded the image then just pastes it into the box after you clear out the default.
Set any of the other below option that you need to set in order to control if the background image will repeat, stay fixed or scroll etc.
Background Image Repeat Status
If using image as background for your pages select image repeat status.
Background Image top/bottom align
If using image as background for your pages select the top/bottom alignment if needed.
Background Image left/right/center align
If using image as background for your pages select the left/right/center alignment if needed.
If using image as background for your pages select the background attachment setting if needed.
Adjust the other settings under Page Body CSS Settings as needed then click the Update Options button.
That should do it.