How to add a random image to a post from gallery and only show one?

You should use the 'orderby' => 'rand' parameter for the get_children() attachments function.

For instance:

$images = get_children( array(
    'orderby'        => 'rand',       // this is random param
    'post_type'      => 'attachment',
    'post_mime_type' => 'image',
    'post_parent'    => get_the_ID(),
);