I noticed someone has opened this thread, but some of the code was removed.
Here is my take on this:
It will make the star clickable, and change it's color depends on the state (Gmail style ;)
first you'll need to put the grey star (or whatever) in wp-content\plugins\wp-favorite-posts\img
then add this function (by silfani): wpfp_before_link_img_grey()
then change wpfp_link_html() like this: http://pastebin.com/LKSJf1Dq
Hope that helps.
Let me know if you have any questions.
And a huge thanks to Huseyin Berberoglu for this great plugin