Image preview size

Usage:
```php
'image' => array(
	'name'	=> 'image',
	'label'	=> 'Image',
	'type'	=> 'image',
	'args'	=> array(
		'size'	=> 'large'
	)
)
```
or array :
```php
'image' => array(
	'name'	=> 'image',
	'label'	=> 'Image',
	'type'	=> 'image',
	'args'	=> array(
		'size'	=> array( 100, 100 )
	)
)
```
if you want use custom image sizes, add this filter:
```php
add_filter( 'image_size_names_choose', function( $sizes ) {
	$possible_sizes = get_intermediate_image_sizes();
	unset( $possible_sizes['full'] );
	if( !empty( $possible_sizes ) )
		return $possible_sizes;
	else
		return $sizes;
} );
```