How to show images below a select box according to the value? Let's say I have a select box with values : 1, 2, 3, 4, 5, 6. Every value has it's own image, which I want to display as a description below the select field. Any Drupal / AJAX way to show this?

By the way this is NOT a custom form. This is a node creation and edit form.
As far as I know there are no ready made solutions for this, and you will need to write a custom module. The way to do this would be to implement hook_form_alter to modify the select element and add a #ajax property. 

If you've never written a module with hook_form_alter then this article might be of help: very-introduction-drupals-hookformalter

And here are some relevant questions on Drupal answers: 


adding-ajax-to-existing-node-form
how-to-use-ajax-in-form-alter-in-drupal-7