Clearing data from memory

i have a thumbnail gallery that changes the thumbnail randomly with fade in and out with alpha. my problem is that after about a minute it starts to slow down. so i use **BitmapData **and i tried to use dispose() but it didnt work. since i never used it before i dont know how to fix my problem:
i am creating the thumbnail with this:(my thumbnails are in the library with linkage)

function picA0() {
 this.createEmptyMovieClip("targetClip0", this.getNextHighestDepth());
 targetClip0._x = 22;
 targetClip0._y = 13;
 i = random(j);
 var pict:BitmapData = BitmapData.loadBitmap("pic"+i);
 this.targetClip0.attachBitmap(pict, 1);

and from a MC that works as the timer i start a function that dissolves’ like this:

function fadeOut(param) {
 this.onEnterFrame = function() {
  if (_root["targetClip"+param]._alpha>0) {
   _root["targetClip"+param]._alpha -= 10;
   _root["targetClipB"+param]._alpha += 10;
  } else {
   _root["pic"+param+"state"] = false;
   delete onEnterFrame;

i tried to add a dispose after the else but its not doing anything