div.dokuwiki form.btn_show .button {
  padding: 1px 0 1px 16px;
  background: transparent url(images/page.png) 0px 1px no-repeat;
  border: none;
}

One thing that became apparent is that IE7 has absolutely no idea how to render this. The correct rendering (as produced by Firefox 3 and all the other browsers) is this:

IE7′s take on rendering this looked more like this:

After a bit more experimentation, I came up with the following illustration which roughly shows what is happening:

The nice thing is that this is fixed in the upcoming IE8, so I’ve taken the stress-free approach; since it’s a relatively “minor” visual bug, there’s no sense it tearing my hair out trying to make it render correctly everywhere. On this occasion I’ve decided that people can either deal with the fact their browser is behind the times, or upgrade, or wait for the upgrade to happen for them.