Your css is doing it. Replace following css if not being used anywhere else. If you’re not comfortable then ask your designer to do it
line 1213
.thumb-box {
position: relative;
width: 100%;
height: auto;
}
line 2321
.box img {
-webkit-transition: all 0.3s ease;
-moz-transition: all 0.3s ease;
-o-transition: all 0.3s ease;
-ms-transition: all 0.3s ease;
transition: all 0.3s ease;
image-rendering: optimizequality;
width: 100%;
height: auto;
}