For example, this will start with the Advicenators logo and switch to the Google logo when hovered over:
<img src="http: //www.advicenators.com/img/hsingle.gif" onmouseover="this.src='http: //img0.gmodules.com/ig/images/logos/approved/white.png'" onmouseout="this.src='http: //www.advicenators.com/img/hsingle.gif'">
If you want to test that example, make sure you remove the spaces in the URLs. I put them there to prevent them from turning into links.
It's also possible to do this using CSS, but it's longer and there are a lot of workarounds. Search to get instructions:
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