Confused with "_on", "_off", "_half"

First of all thank you for this nice little plugin. I love it!

I need some help. I use some custom plus-minus images and I want them both to be “_on” when the user didn’t vote yet and both to be “_off” when he voted. Is that possible?

Thank you!

https://ww.wp.xz.cn/plugins/wp-postratings/