var CoverUps = {
			initialized: false,
			todo: [],
			initialize: function(thumbnails){
				thumbnails.each(function(i){
					var image = new Image();
					image.onload = function(){
						coverup = $('project' + i.id + '_cover-up');
						if(coverup){
							new Effect.Fade(coverup);
						}else{
							CoverUps.todo.push('project' + i.id + '_cover-up');
						}
					}
					image.src = i.image;
				});
				Event.observe(document, 'dom:loaded', CoverUps.fadeLeftovers);
			},
			fadeLeftovers: function(){
				CoverUps.todo.each(function(e){
					new Effect.Fade(e);
				});
			}
		};

needs to be modified to be more general purpose.

Loading Elements for Images

JavaScript