How to make rollover images

I see them everywhere, rollover images. Like the "New Topic" buttons, or the "New Reply" buttons. I would like to know how to do that. I find the codes all over the place, but how do I use them? I will admit, I am code illiterate. I do not know any code(other than BBcode). So if you know how to use them, please help me! Thank you so much (:

Regards,
Harper