All on-page image blocks:
.intrinsic:hover img {filter:grayscale(1)}

Card Layout
.design-layout-card:hover img {filter:grayscale(1)}

Overlap Layout
.design-layout-overlap:hover img {filter:grayscale(1)}

Collage Layout
.design-layout-collage:hover img {filter:grayscale(1)}

Stack Layout
.design-layout-stack:hover img {filter:grayscale(1)}

Poster Layout
.design-layout-poster:hover img {filter:grayscale(1)}