Not really enough information here. Image you are hired to file defects on an application you are testing. The developers are located in a completely different. If they had to ask a question, you'd answer it, they'd have another question, you'd answer that, etc. and the turn around time is one message a day. At that rate it might take days to solve the problem. This is sort of what posting a request for help here is like. The only difference is that no one is paid to help you.
For me to know the answer means I would be able to automate the task as well. Whatever you need to automate the task is what I need to automate the task (i.e. answer the question).
What is ele? is it the BODY tag? Is it a DIV? What is the error you are getting? Why are you casting the return value to a String? It should already be returning a String. Maybe it is an environment issue and nothing to do with WebDriver or your code.
If I use the following it works:
driver.get("
http://www.scuba.com");
WebElement body = driver.findElement(By.tagName("body"));
String actual = body.getCssValue("background-image");
String expected = "url(\"
http://www.scuba.com/_homepage2012/images/fall-theme-background.jpg\")";
assertEquals(expected, actual);
Hopefully this will help you with what you are doing differently. You can compare the site's source to your site's source.