It's as easy a sthis:

<img
    src="img/cmsimage.php/[PRIO]552[/PRIO][PRIO_ELSE]264[/PRIO_ELSE]x0x8/{THUMB_HASH}.{THUMB_EXT}"
    data-grayscale="img/cmsimage.php/[PRIO]552[/PRIO][PRIO_ELSE]264[/PRIO_ELSE]x0x8x0xGS/{THUMB_HASH}.{THUMB_EXT}"
    alt="[CAPTION]{CAPTION}[/CAPTION][CAPTION_ELSE]{IMAGE_NAME}[/CAPTION_ELSE]"
    class="image-grayscale"
/>