[[Image:Wiki.png|50px|link=MediaWiki]]

Note: The code above will work in mediawiki version 1.14 and above only
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Here is the workaround for older versions
Edit the Common.css

.imagelink_somename a {
  width:100px;
  height:100px;
  display:block;
  text-decoration:none;
  background-image: url("http://fullurltoimage")
}

In your wiki code:

<div class="imagelink_somename">[[Some link|&nbsp;]]</div>

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Or try the quick way

<div style="position: relative; width: {{{width}}}; height: {{{height}}}; overflow: hidden;">
<div style="position: absolute; top: 0px; left: 0px; font-size: 100px; overflow: hidden; line-height: 100px; z-index: 3;">[[{{{link}}}|&nbsp;&nbsp;&nbsp;]]</div>
<div style="position: absolute; top: 0px; left: 0px; z-index: 2;">[[Image:{{{image}}}|{{{width}}}|{{{link}}}]]</div>
</div>

More workarounds found at mediawiki docs
http://www.mediawiki.org/wiki/Help:Linked_images#link.3D_Syntax_available_since_version_1.14

$wgUrlProtocols = array( 'http://', 'https://', 'mailto:' ,'notes://');