If yoour code is in a class you could preload the images by using the Ext.create('Ext.Img', .... in the constructor and saving the objects in an array into the class. You can show them via this.images.show(); for example.
But this means you can't use your html any more. You would have to use the item property. I hope you get what I mean xD
If not, I can show you an example...
var image = new Image(100,100) //100, 100 is the size