Welcome to WebmasterWorld nate4cfdev!

Is there any danger in preloading and showing the full size images to search engines, but not the user using css display:none? Is this breaking any rules?

Absolutely none at all. The search engine only sees what the browser sees. Those hidden images represent a user functionality.

I'd have to ask why you would want to preload those larger images? If there are quite a few of them, it could take a bit for that page to load for the user.

The reason for preloading the main images is so that they show up instantly when the user clicks on a thumbnail. Without preloading them, there's a delay each time the user clicks on a thumbnail and so the main image does not instantly load, but it takes a second or two before it loads. Yes, it takes longer for the page to load initially, but it makes for a better user experience after that.

nate4cfdev, I would load the images only when clicked.

I'd rather wait two seconds each time than 20 seconds before the page loads. If the page is loaded and the visitor clicks, you already grabbed him/her, whereas you may lose them if it takes too long to load.

You could make the 'wait' for the image loading less 'visible' by playing a small animation (loading...) that you would have preloaded :-)

just my £0.02