Create a file named <dokuwiki>/conf/userstyle.css and add the following CSS to hide the link images for all links (remove the CSS classes for links you want to keep the images):
a.urlextern, a.interwiki, a.windows, a.mail, a.media { padding-left: 0; background: none !important; }
You may need force a re-cache of the CSS. To do that either use CTRL+SHIFT+R in firefox or on Linux do a $> touch <dokuwiki>/conf/dokuwiki.php.
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