Re: Background Image
Eatlon's body background image is specified in eatlon.css as
body { background: url("/styles/eatlon/body_bg.gif") repeat-x #9FAEC2; }
so this is what needs to be edited or overridden. You could add a custom rule in the Look and Feel admin page's custom CSS textarea, or add a "custom.css" file in the styles/eatlon/ directory. These would be upgrade-proof. Or you could edit styles/eatlon.css itself but would have to repeat the edit when you update your site if eatlon.css is replaced.
I don't know why your attempts so far didn't work. I haven't tried Tiki's built-in CSS editing for a while, but surely FTP (replacing eatlon.css or adding a custom file) should work. Be sure to clear the caches (at tiki-admin_system.php) after making the change. Sometimes the browser cache needs to be cleared, too, especially if the replacing image file has the same name as the replaced file.
-- Gary