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    Solved on my own - just seach for
    <block type="page/html_head" name="head" as="head">

    in app/design/frontend/default/hellomix/layout/page.xml

    <action method="addItem"><type>skin_js</type><name>js/imgpreview.js</name></action>

    clear caches and check your sourcecode when reloading the frontend if
    <script type="text/javascript" src=""></script>
    is loaded within the header

    If imgpreview.js is not in your directory skin/frontend/default/hellomix/js you can download it here:

    I´m sure the admin will post another fix soon - but sorry, i am always in a hurry ;)