Thats right, but if you really want to use images, then it is possible to do so with CSS. I am about to write a blog post on this, so I'll explain in more detail there, but if you design an image that contains both the images involved (so for example, the original image on top of the desired rollover image, all in one picture) then you can use the background-position property to create a rollover effect.
For a good example of the use of this, go to Glush.tv, then have a look at the search button at the top.
Main Site
The Image
The CSS
The CSS part is under 'input.button'
Hope it helps, its a great little method 
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