Hi,
You can solve that in CSS by adding this:
body
{
background-image: url('path_to_your__background_image'); /* path to the image*/
background-repeat: repeat; /* repeats the image in X and Y axis */
}
You should have your css file in the template direstory. When you access the page via ftp that will be:
your_wordpress_directory/wp-content/themes/your_theme
I hope that helps,
Wojtek