If you have a question regarding a specific piece of javascript, or how to go about writing something yourself, post it in this forum. If you are looking for a Rollover script which is already written (there are tons), use the
Script Requests forum - let me know if you want this thread moved there.
Otherwise, if you want to write it yourself, start by checking out the
onMouseOver event. You can use that event to determine when to switch the image source.