Centering vertical imgs loaded dynamically

Hello world:)
I have been using a photo gallery that I found here to load images into a movie clip on the stage. My problem is that the vertical images are not centered on the stage. They (properly) load from the upper left corner, but I need them to be centered between my two buttons. How do I keep only one movie clip, as well as maintaining the centering of my horizontal AND vertical images?? Here’s the code I have for the gallery…at the bottom I toyed around with x and y…but haven’t figured it out. Any help would be great:) Thanks:)

 function loadXML(loaded) {

 if (loaded) {

xmlNode = this.firstChild;
image = [];
description = [];
total = xmlNode.childNodes.length;
for (i=0; i<total; i++) {

image* = xmlNode.childNodes*.childNodes[0].firstChild.nodeValue;
description* = xmlNode.childNodes*.childNodes[1].firstChild.nodeValue;

}
firstImage();

} else {

content = "file not loaded!";

}

}
xmlData = new XML();
xmlData.ignoreWhite = true;
xmlData.onLoad = loadXML;
xmlData.load("XML/imageslove.xml");
/////////////////////////////////////
listen = new Object();
listen.onKeyDown = function() {

if (Key.getCode() == Key.LEFT) {

prevImage();

} else if (Key.getCode() == Key.RIGHT) {

nextImage();

}

};
Key.addListener(listen);
previous_btn.onRelease = function() {

prevImage();

};
next_btn.onRelease = function() {

nextImage();

};
/////////////////////////////////////
p = 0;
this.onEnterFrame = function() {

filesize = picture.getBytesTotal();
loaded = picture.getBytesLoaded();
preloader._visible = true;
if (loaded != filesize) {

preloader.preload_bar._xscale = 100*loaded/filesize;

} else {

preloader._visible = false;
if (picture._alpha<100) {

picture._alpha += 10;

}

}

};
function nextImage() {

if (p<(total-1)) {

p++;
if (loaded == filesize) {

picture._alpha = 0;
picture.loadMovie(image[p], 1);
desc_txt.text = description[p];
picture_num();

}

}

}
function prevImage() {

if (p>0) {

p--;
picture._alpha = 0;
picture.loadMovie(image[p], 1);
desc_txt.text = description[p];
picture_num();

}

}
function firstImage() {

if (loaded == filesize) {

picture._alpha = 0;
picture.loadMovie(image[0], 1);
desc_txt.text = description[0];
picture_num();

}

}
function picture_num() {

current_pos = p+1;
pos_txt.text = current_pos+" / "+total;

}
mc = _root.picture
mc._x = (Stage.width/3)-mc._width
mc._y = (Stage.height/6)-mc._height

next2_btn.onRelease = function(){
    gotoAndStop("moments");
};