Well the option you have now is not bad. Because only the images that the user want to see are loaded (the hover image). The drawback is that the image is only loaded when the user hovers over it and triggering the request and then there can be a delay (because the image will be needed downloaded and renderd after that it becomes cache! if you want it so).
The other solution you are giving will cause alot more data usage for you and the user will the user only wanted to see 1 hover image.
So i say stick with what you have now