Create a file named userstyle.css in the <dokuwiki>/conf/ directory 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):

.page a.urlextern,
.page a.interwiki,
.page a.windows,
.page a.mail,
.page a.media {
  padding-left: 0 !important;
  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.

If it won't work you maybe have to replace the CSS-classes above (.page) with .dokuwiki.