How to change airdroid background wallpaper with greasemonkey script

Hi im newbie and very amateur. I want to change background wallpaper of airdroid web page with grasemonkey script but i failed. I found stylish script but i want grasemonkey. This is background wallpaper of airdroid web page code;

<div id="block-wallpaper" origin="http://cdn1.airdroid.com/V2121406161807/theme/stock/images/wallpaper/default.jpg" class="type-scale">
    <img class="stock" src="http://cdn1.airdroid.com/V2121406161807/theme/stock/images/wallpaper/default.jpg">
    <div class="bg" style=""></div>
</div>

please help me :(

Solution

I figured as follows:

setTimeout(function() {

var images = document.getElementsByTagName('img');

    for (i=0; i<images.length; i++)
    { 
        if(images[i].src == "http://cdn1.airdroid.com/V2121406161807/theme/stock/images/wallpaper/default.jpg") 
        { 
            images[i].src = "http://wallpapers.wallbase.cc/rozne/wallpaper-2628182.jpg"; 
        } 
        i=i+1; 
    }   }, 600);