Use Mutation Observers to place image placeholders (spinners) on loading images within a target element.

spinner.js

//whenever uncached images are added to the dom tree within target,
//replace them with a spinner gif until they're loaded
function replaceImages(target, spinner_src) {
    var spinner, observer;

    //detect support
    if (window.MutationObserver) {
        //preload spinner (probably a gif)
        //if it's not ready for first use, oh well
        spinner = new Image();
        spinner.src = spinner_src;

        //yay, setup callback for mutations
        observer = new MutationObserver(function (mutations) {
            //for every mutation
            mutations.forEach(function (mutation) {
                //for every added element
                mutation.addedNodes.forEach(function (node) {
                    // Check if we appended a node type that isn't
                    // an element that we can search for images inside,
                    // like a text node.
                    if (typeof node.getElementsByTagName !== 'function') {
                        return;
                    }

                    var imgs = node.getElementsByTagName('img');
                    //for every new image
                    imgs.forEach(function (img) {
                        var parent, spinner_clone, onLoadImage;
                        //if it hasn't loaded yet
                        if (!img.complete) {
                            //remeber what it's parent is
                            parent = img.parentNode;
                            //clone our preloaded spinner gif
                            spinner_clone = spinner.cloneNode();
                            //replace the not-yet-loaded image with a spinner
                            parent.replaceChild(spinner_clone, img);

                            onLoadImage = function (event) {
                                //the image has loaded so replace the spinner back with the image
                                parent.replaceChild(spinner_clone, img);
                                //this event will never fire again. might as well clean it up
                                event.target.removeEventListener('load', onLoadImage);
                            };
                            //when the image is done loading, call the function above
                            img.addEventListener('load', onLoadImage);
                        }
                    });
                });
            });
        });

        //bind mutation observer to a specific element (probably a div somewhere)
        observer.observe(target, {childList: true, subtree: true});
    }
}