How to do...uhmm...seperate image rollover??

Hi, I forgot where I saw this,but I was wondering how to do a rollover not over a roll over image,but somewhere else on the page.

So like if Y was a pic,and you wont over Y, instead of rolling over the Y pic ,it would do a seperate rollover where X is..

Example:

Y Rollover shows up here -> X

Thats the only way I can explain in text.I am sure someone here knows what its called,but I forgot.

call image Y X then:

great. Is that better now?

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Would make
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change to hiphip2.gif because of the name.
It doesn't matter where it is located.