hover over image

First time using siteorigin….

I put the following css code in attributes:

.so-widget-sow-image img {
-webkit-transition: all 250ms ease-in-out;
-moz-transition: all 250ms ease-in-out;
-ms-transition: all 250ms ease-in-out;
transition: all 250ms ease-in-out;
}

.so-widget-sow-image img:hover {
-webkit-filter: brightness(50%);
-moz-filter: brightness(50%);
-ms-filter: brightness(50%);
filter: brightness(50%);
}

What do I enter for css class.

I tried so-image-container