It's pretty easy to rip images and sprites from any game, especially since there are tools to capture the screen and then "modified pixels", to single out animated objects.
In terms of regular players getting a copy of your game, maybe put a special thanks in the credits that stands out and says "Thank you all for purchasing the game and supporting my passion". Nice people generally will buy things if they have the money <img src="smileys/smiley17.gif" border="0" align="middle" />
However, stopping people from cloning your game is tricky. I guess the best to hope for is getting a copyright to at least stop people from reselling your game under their own name (and include your website in the game art, as well as maybe a text or code) so that people always know who really created it.