WordPress does not handle monochrome JPEG image correctory. This restriction is described in "file_is_displayable_image() function at /wp-admin/includes/image.php".
WordPress will not create meta data information for monochrom JPEG images.
This behavior brings some serious troubles in '[gallery]' shortcode thumbnails.
Most of modern WEB browsers can display monochrom JPEG images, please remove this restriction.
[/wp-admin/includes/image.php]
// is the file an image suitable for displaying within a web page?
function file_is_displayable_image($path) {
$info = @getimagesize($path);
if ( empty($info) )
$result = false;
elseif ( !in_array($info[2], array(IMAGETYPE_GIF, IMAGETYPE_JPEG, IMAGETYPE_PNG)) )
// only gif, jpeg and png images can reliably be displayed
$result = false;
elseif ( $info['channels'] > 0 && <strong><em>$info['channels'] != 3</em></strong> ) {
// some web browsers can't display cmyk or grayscale jpegs
$result = false;
}
else
$result = true;
return apply_filters('file_is_displayable_image', $result, $path);
}