Javascript-Setting background image of a DIV via a function and function parameter

You need to concatenate your string.

document.getElementById(tabName).style.backgroundImage = 'url(buttons/' + imagePrefix + '.png)';

The way you had it, it's just making 1 long string and not actually interpreting imagePrefix.

I would even suggest creating the string separate:

function ChangeBackgroungImageOfTab(tabName, imagePrefix)
{
    var urlString = 'url(buttons/' + imagePrefix + '.png)';
    document.getElementById(tabName).style.backgroundImage =  urlString;
}

As mentioned by David Thomas below, you can ditch the double quotes in your string. Here is a little article to get a better idea of how strings and quotes/double quotes are related: http://www.quirksmode.org/js/strings.html

From what I know, the correct syntax is:

function ChangeBackgroungImageOfTab(tabName, imagePrefix)
{
    document.getElementById(tabName).style.backgroundImage = "url('buttons/" + imagePrefix + ".png')";
}

So basically, getElementById(tabName).backgroundImage and split the string like:

"cssInHere('and" + javascriptOutHere + "/cssAgain')";