How to theme a radio button field on node edit form? The standard way the radio buttons field is rendered on a node add/edit form is that there is a wrapper div with classes:

form-item form-type-radio form-item-mymodule-imglayout-und

and inside there are input and label elements. Where to hook to change this? I would like the wrapper div to have classes: 

form-item form-type-radio form-item-mymodule-imglayout-und mycustomclass

And the custom class should be dependent on the option the button represents, or just be sequential: first, second, third, ...etc

Additional info:

My point is to be able to add a distinct big background image to each radio button, depending on the option it represents. Like below - note that each radio button has an associated image, representing its option visually.
I think what you need to do is just altering the form array. You can accomplish this in your own hook_form_alter implementation. 

For example:

function YOURMODULE_form_alter(&amp;$form, &amp;$form_state, $form_id) {
    if($form_id == 'YOUR_CONTENT_TYPE_node_form') {
        $form['YOUR_ELEMENT_NAME']['#attributes']['class'][] = 'your-custom-class';
    }
}