pada style.css:
/* =Image to Greyscale
----------------------------------------------- */
.bw {
    -webkit-filter: grayscale(1) !important;
    -moz-filter: grayscale(1) !important;
    -ms-filter: grayscale(1) !important;
    filter: grayscale(1) !important;
}


pada konten:
<img class="bw" src="..."/>