I know exactly what you mean. I had to do a project for this a while back in class.
What you have to do is set an array. Well, never mind. I'll just write one for you.This will be in javascript. So put the < script > tags around it.
To quickly explain this, the top part is an array. Change only the red parts. Don't change the 6 unless you have too. That number HAS to be one more then the last array you have. So, if you have changeIMG[6] the 6 in the new Array part HAS to be 7. The reason for that is javascript starts at 0.
The blue url's should be the same. This is the main image that will be seen first, and when you mouse out. Take this code and run with it. Let me know if it doesn't work.