Follow the css:
.parceiros li a img{
margin-top: 5%;
-webkit-filter: grayscale(100%);
-moz-filter: grayscale(100%);
-ms-filter: grayscale(100%);
-o-filter: grayscale(100%);
}
.parceiros li a img:hover{
-webkit-filter: grayscale(0);
-moz-filter: grayscale(0);
-ms-filter: grayscale(0);
-o-filter: grayscale(0);
}