JavaScript Image Preloading Script
Not really about performance, but this is more about clarity of the code.


Attach onload handlers first, for readability. Appending the src starts the loading process. If you saw code that loaded something, expect to do something, but haven't found the handler to do it makes it a bit confusing.
Declare one handler function in a persistent scope and have the onload refer to it, rather than create one handler for each onload, or creating the function everytime the whole routine executes.
Instead of find(), use jQuery function's second parameter, context. With this, jQuery finds the elements with the given selector under the jQuery object that is on the second parameter.
If the images are children of data.content, use children() instead of find() to avoid deep DOM traversal. Also, the lower you are in the DOM, the less deep find() has to dive and find your elements. 
Use strict comparison as much as possible.
Don't forget to declare the variables and use var. You'd be declaring globals if you didn't.


And so:

var $images = $('img', data.content),
  imagesLength = $images.length,
  imageLoadingIterator = 0,
  img, i;

function onImageLoad() {
  if (++imageLoadingIterator === images.length) _continue();
}

for (i = 0; i &lt; imagesLength; i++) {
  img = new Image();
  img.onload = onImageLoad;
  img.src = $images[i].src;
}


However, these advices might have a very negligible gain in performance, and might be unnecessary since the compiler might be optimizing them in some way.