All on-page image blocks:
.intrinsic:hover img {opacity: 0.75 !important;}
 
Card Layout
.design-layout-card:hover img {opacity: 0.75 !important;}
 
Overlap Layout
.design-layout-overlap:hover img {opacity: 0.75 !important;}
 
Collage Layout
.design-layout-collage:hover img {opacity: 0.75 !important;}
 
Stack Layout
.design-layout-stack:hover img {opacity: 0.75 !important;}
 
Poster Layout
.design-layout-poster:hover img {opacity: 0.75 !important;}