CSS code to apply to MediaWiki:Wikia.css:
This will allow the image to be translucent to start with, and turn totally solid upon hover. Example coding:
<span class="hoverimage">[[File:Example Popup.png]]</span>
This was originally designed for Isle of Tune Wiki, but can be applied to other wikis.
This can also be applied to the wiki wordmark like this, such as on this page. CSS to apply: