Here it is: Simple Image Pre-loader
Use the new template tag, <PRELOAD_IMAGE_NAME> to grab the previous image name for pre-loading.
img1 = new Image();
img1.src = "images/<PRELOAD_IMAGE_NAME>";
Note, if the last image has been viewed, the addon will go back and use the first image name.
This may be useful if you have the wraparound image addon installed.
If you know what you're doing, feel free to modify the addon to your liking.