That is assumedly because the first folder Imagevue finds in your gallery, happens to be a passworded folder. You can manually edit the default Facebook share picture though ... Open the file
iv-includes/templates/default/index.gallery.phtml, line 23:
<meta property="og:image" content="<?php echo pageURL().htmlspecialchars($this->firstImage->thumbnail); ?>?<?php echo htmlspecialchars($this->firstImage->getThumbnailMTime()); ?>" />
... change to ...
<meta property="og:image" content="http://domain/path/file.jpg" />
Simply add an absolute path to the image on your web server. It does not have to be a thumbnail, in fact its better if its not because Facebook recommends minimum 200 px width/height.