Create thumbs only when a post is sticky or any other way?

As i understand you want new thumb size only for sticky post and for rest is same

1).define a new size using add_image_size() and use is_sticky() to check post is sticky or not

add_image_size('thumb-sticky','your_width','you_height')

2.)and put if condition in your slider or whatever

if( has_post_thumbnail() ):
  //check post is sticky 
  if( is_sticky() ):
    //This post is sticky print new thumbnail size i.e 'thumb-sticky'
    the_post_thumbnail('thumb-sticky');
  else:
    //This post is not sticky print thumbnail.
   the_post_thumbnail();
  endif;
endif;

EDIT:

As you want to image size specific to sticky post then do as in above code and paste below code in functions.php

add_filter('intermediate_image_sizes','ravs_thumb_sticky');
function ravs_thumb_sticky($image_sizes){
    global $post;
    if( ! is_sticky( $post->ID ) )
        $image_sizes= array_values( array_diff( $image_sizes, array('thumb-sticky') ) );
return $image_sizes;
}

Above code simply remove image size thumb-sticky if your post is not sticky post , hence for this size thumbs not generate for post.

Important Link:

intermediate_image_sizes

or you can use

intermediate_image_sizes_advanced