I can’t add hover effect to my image gallery

Hi,
I would like to make a hover effect like a small bounce when mouse is hovered on to the image .is it possible to do that?

Should I paste his code somewhere? I Don’t know where. Could you help me please?

.sow-image_darken .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;
}

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