Hello
if you are using style 1
from theme-assets/css/style-1.css
line no 694
If you are using Style 2
Change this from theme-assets/css/style-2.css
line no 713
https://puu.sh/qh6Kq/48ed085cea.png
Change this like
.partner a img {
background-color: #ececec;
width: 25%;
float: left;
padding: 1px;
}
.partner a:hover img {
}