When I set the background image, after reloading or changing page, the image disappears, it also happens in this demo on your site!
[attachment:32]C:\fakepath\demo.png[/attachment]

[attachment:32]C:\fakepath\demo.png[/attachment]

[attachment:32]C:\fakepath\ reload.png[/attachment]

[attachment:65]C:\fakepath\original.png[/attachment]

Dear giumarto,
Do you change it in Cpanel? It is demo and it will dissappear when you delete cookies.
prntscr.com/9rij8n
Please change it in admin site: Extensions-> Templates -> your_template

Thanks.