Okay I would set it up like this
First your RefrenzenPage.php which will act as your ReferenzPage Holder
http://pastie.org/2237295
Then your RefrenzPage.php which will hold your RefImages
http://pastie.org/2237298
Followed by the RefImage.php which will serve to hold multiple Images. This way you don't need to worry about blank fields, it will loop through existing ones wether you have 1 RefImage or 100
http://pastie.org/2237304
Then in your RefrenzenPage.ss you could call all preview images from the ReferenzPages underneath it. Since the PreviewImage is held on the ReferenzPage you can just call it with Children to build your menu
<% control Children %>
<a href="$Link">$PreviewImage</a>
<% end_control %>
and then in Referenz.php you could loop though all the RefImages with the following.
<% control RefImages %>
$Attachment
<p>$Leistung</p>
<% end_control %>
Hopefully I'm not missing any commas or semicolons anywhere :)