Hello,
Actually, you can simulate that first click, so the "Properties" panel will be collapsed by default. The file you need to edit is 
~\CMSModules\MediaLibrary\Controls\MediaLibrary\MediaLibrary.ascx and this file contains a block of javascripts, so please just add there the following code:
$j(document).ready(function () {
        $j('.DialogResizerArrowV').click();
    });
Best regards,
Jan Hermann