Simple If Statement

I just need to change this one line of code to include an if statement to see if their is a value for “pm_menu_image_meta”. I just can’t figure it out.

$output .= '<div class="vc_single_image-wrapper vc_box_shadow_3d vc_box_border_grey"><span class="spg-clip"><img src="'.get_post_meta($id, 'pm_menu_image_meta', true).'"/></div>';