The Harry Potter Wiki uses an external provider for their main page poll, and arrow.wikia.com uses the standard poll extension with additional CSS customization. If you're looking for specific ways to customize polls, let us know and we'll see what we can do.
Thank you!
Is it possible to add this picture to the background?
Maybe, if you look at w:c:arrow:MediaWiki:Common.css and scroll down to /**Poll**/ the very first thing they do is a background image.