To add custom icon for custom post type .. you can use this function for that..
// Add this to functions.php file..
When you are going to create custom post type then by default icon code will be like this..
'menu_icon' => 'dashicons-cart',
You can also add the full URL of an image file you want to display as icon, like this: you can change path..
'menu_icon' => 'http://www.nstplaent.com/wp-content/uploads/2017/04/nst-icon.png',
You can upload image with size around 22 x 22 and upload there in dashboard media or in theme images folder.
Also can adjust or align icon by using admin css function.
Thanks dude..