Movieclip dimmensions

I have this code that uploads thumbnails and once they are clicked they get enlarged
var whichImage = 16;
var thumb:Array = new Array();
var largepic:Array = new Array();
var titles:Array = new Array(“Biron”, “Icon”, “Marin”, “Vincent”, “Ioana”, “LiForms”, “Landscape with figure”, “Maternity1”, “Musician”, “Mother’s Pain”, “Zoomahia”, “Embrace me…”, “Charlie”, “Three Figures”, “Henry Lee”);
loadmovies();
function loadmovies() {
for (i=1; i<whichImage; i++) {
thumb* = attachMovie(“emc”, “imageLoader”+, i);
thumb
._x = (i-1)60;
thumb
._y = 490;
}
}
var mclListener:Object = new Object();
mclListener.onLoadStart = function(target_mc:MovieClip) {
target_mc._height = 50;

};
mclListener.onLoadComplete = function(target_mc:MovieClip) {
target_mc._alpha = 50;

};
mclListener.onLoadInit = function(target_mc:MovieClip) {
target_mc.onRollOver = function() {
this._alpha = 100;
};
target_mc.onRollOut = function() {
this._alpha = 50;
};
target_mc.onRelease = function() {
for (i=1; i<whichImage; i++) {
var x = “imageLoader”+;
if (this._name == x) {
loadMovie(“large/picture”+
+".jpg", “bigimage”);
var count = i-1;
txt.text = titles[count];
}
}
};
};
function init() {
for (i=1; i<whichImage; i++) {
var image_mc:MovieClipLoader = new MovieClipLoader();
image_mc.addListener(mclListener);
image_mc.loadClip(“large/picture”++".jpg", “imageLoader”+);
}
}
init();
onEnterFrame = function () {
for (i=1; i<whichImage; i++) {
var mousex = _xmouse/(Stage.width/4);
if (_ymouse>=520) {
thumb*._height = 30;
thumb*._width = 30;
if (mousex<2 and thumb[whichImage-1]._x>Stage.width/8) {
thumb*._x -= .5;
thumb*._y = 490;
} else if (mousex>2 and thumb[whichImage-(whichImage-1)]._x<Stage.width/8) {
thumb*._x += .5;
thumb*._y = 490;
}
}
}
};

the problem is i cannot set height and width for the thumbs i upload. the _height and _width properties work fine after if (_ymouse>=520) but if i try to set the properties before i get a blank page. So my question is, what can I do to set the height and width values, and why is it working after the if statement and not before?
Thanks