OK, here's a solution on how to center ZX Slideshow for all those interested:

Go to includes/templates/YOUR_TEMPLATE/css/index_home.css and find lines 90-97. Originally, this is how it looks:
Code:
.theme-default .nivoSlider {
    position:relative;
    background:#fff url("../images/loading.gif") no-repeat 50% 50%;
    margin-bottom:50px;
    -webkit-box-shadow: 0px 1px 5px 0px #4a4a4a;
    -moz-box-shadow: 0px 1px 5px 0px #4a4a4a;
    box-shadow: 0px 1px 5px 0px #4a4a4a;
}
Now, simply change the code in red with margins of your choice. For those who don't know, the numbers represent "top right bottom left". Change it to something like this if you want the slideshow to be centered:
Code:
.theme-default .nivoSlider {
    position:relative;
    background:#fff url("../images/loading.gif") no-repeat 50% 50%;
    margin:0 auto 50px auto;
    -webkit-box-shadow: 0px 1px 5px 0px #4a4a4a;
    -moz-box-shadow: 0px 1px 5px 0px #4a4a4a;
    box-shadow: 0px 1px 5px 0px #4a4a4a;
}
This is it, your slideshow should now be centered.