Alexandru,
The OxWall page is full of DIVs over DIVs over DIVs... etc...
Almost each DIV has a background color defined in the CSS, which will be "in front" of your background image if you have attached it to the body tag. Have you tried changing those colors to "transparent"? You can do it editing the CSS of your theme or via Admin panel, in the Appearance item.
Take a look on the CSS file and you will see ehat I am talking about. :)
Let me know if you still need help after all.
Julio Marchi
jcmarchi@gmail.com