how to display data from user panel using cmb2 metabox?

by

According to CMB2 documentation, instead of

<img src="<?php echo get_post_meta( get_the_ID(), 'author_sign', true) ?>" alt="">

you should use something like

<?php echo wp_get_attachment_image( get_post_meta( get_the_ID(), 'author_sign', true ), 'medium' ); ?>

or

<img src="<?php echo wp_get_attachment_image_url( get_post_meta( get_the_ID(), 'author_sign', true ), 'medium' ); ?>" alt="">