Greasemonkey script alert box issue

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Javascript

Hi I'm new to CodeProject. So Hello Everyone!

I'll just cut to the chase. I started learning JavaScript on my own recently and I thought of learning it by doing mostly instead of only reading tutorials and books. So I started writing a Greasemonkey script for Facebook.

Basically what it does is it creates a link to download photo albums from Facebook. I'm far from finished but I seem to have hit a snag. Here's my code so far,

The format of the HTML code block where the URLs of pics are.

<div class="tagWrapper">
<i style="background-image: url(https://fbcdn-photos-a.akamaihd.net/hphotos-ak-ash4/299595_10150290138650735_543370734_8021370_355110168_a.jpg);"></i>
</div>

And here's my script so far,

var picURL;
var modURL;
var URLset = [];

for(var i=0; i<picURL.length; i++)
{
    //gets all the URLs in the page
    picURL = document.getElementsByClassName("tagWrapper")[0].children[0].style.backgroundImage;
    modifyURL(picURL);
    URLset.push(modURL);  //inserting them to an array
    
    alert(URLset) ;
}

function modifyURL();
{
    //doing the replacements to make them downloadable
    modURL = picURL.replace("url(", "").replace(")", "");
    modURL = modURL.replace("a.jpg", "n.jpg");
    var dwn = "?dl=1";
    modURL = modURL.concat(dwn);
    
    return modURL;
}

I have created a fiddle so that it all would be clearer.

Since I'm half way through it, I just want to make sure it works up to this point so I have put an alert box to show all the modified URLs stored in the array. (supposed to, at least) the problem is, it isn't showing up.

Can anyone please tell me what I have done wrong here? Bare with me for I'm new to JS as I've mentioned earlier. Any help in the right direction would be much appreciated.

Thanks in advance.