Hover effect on portfolio thumbnails

Hi there
I want to create a hover / roll-over effect on the thumbnail/featured images for my portfolio. No animation, just a change from a colour image to a slightly desaturated image. Is it possible to edit that in the Additional CSS section or do I have to install a plugin? This is the code I’ve tried, but it did nothing. Am I applying it to the wrong item, or is the code wrong, or both?

.single-jetpack-portfolio .entry-thumbnail img: hover {
filter: grayscale (60%);
-webkit-filter: grayscale (60%);
}
It for the site: kingsundew.co.za