Display ajax preloader for large images within posts

How you said one option is to place a preloader while the image is loading.

If you cannot touch so deep with JavaScript and PHP I have one simple sugestion which I think will work for you.

<div class="image-preloader">
   <img src="https://wordpress.stackexchange.com/questions/129186/yourimage.jpg" alt="" />
</div>

Create a wrapper (holder) around your image.
With CSS put the preloader as a background on the HOLDER DIV.

.image-preloader {
  background('images/loader.gif') 50% 50%;
}

Like that while the image is loading you will have your preloader and when the Image is loaded it will be hidden under it.