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<div class="bilder">
  <div><img src="http://www.gegen-bilderklau.net/images/avatars/avatar-41691.gif" alt="" /></div>
  <div><img src="http://www.gegen-bilderklau.net/images/avatars/avatar-41691.gif" alt="" /></div>
  <div><img src="http://www.gegen-bilderklau.net/images/avatars/avatar-41691.gif" alt="" /></div>
  <div><img src="http://www.gegen-bilderklau.net/images/avatars/avatar-41691.gif" alt="" /></div>
  <div><img src="http://www.gegen-bilderklau.net/images/avatars/avatar-41691.gif" alt="" /></div>
  <div><img src="http://www.gegen-bilderklau.net/images/avatars/avatar-41691.gif" alt="" /></div>
  <div><img src="http://www.gegen-bilderklau.net/images/avatars/avatar-41691.gif" alt="" /></div>
  <div><img src="http://www.gegen-bilderklau.net/images/avatars/avatar-41691.gif" alt="" /></div>
  <div><img src="http://www.gegen-bilderklau.net/images/avatars/avatar-41691.gif" alt="" /></div>
  <div><img src="http://www.gegen-bilderklau.net/images/avatars/avatar-41691.gif" alt="" /></div>
  <div><img src="http://www.gegen-bilderklau.net/images/avatars/avatar-41691.gif" alt="" /></div>
  <div><img src="http://www.gegen-bilderklau.net/images/avatars/avatar-41691.gif" alt="" /></div>
  <div><img src="http://www.gegen-bilderklau.net/images/avatars/avatar-41691.gif" alt="" /></div>
  <div><img src="http://www.gegen-bilderklau.net/images/avatars/avatar-41691.gif" alt="" /></div>
</div>

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.bilder {
  clear: both;
  width: 540px; /* SIEHE A) */
}
.bilder div {
  float: left;
  width: 100px; /* SIEHE B) */
  border: 1px solid black;
  margin-left: -1px;
  margin-top: -1px;
  text-align: center;
  padding: 3px;
}
.bilder div img {
  margin: 0px auto;
  width: 100px; /* SIEHE B) */
}