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.