[Plugin: Hover Image] How to hover image to become darker?

Hi,

I am new in using wordpress. This plugin was setup by my web designer.

he gave me this code
a.hover_img:hover img { opacity:0.6; }

If i change the opacity 0.6 to lower, it will fade brighter but not darker. I did change the figure to higher, but its doesnt change any effect.

Any idea how can I make my button hover darker?

here is my website link, please do have a look.
https://www.prforaustralia.com/cn

Thanks.

https://www.ads-software.com/extend/plugins/hover-image/