Up to eight pictures routed into one, steered by a ninth in greyscale.
The control picture's brightness is divided into bands, and each band shows a different source. Darkest areas show the first, next darkest show the second, and so on up to white.
The bands do not meet at hard lines. Each boundary cross-fades into the next, so the handover between sources follows the control picture's own gradients. That is the difference between this and a posterised stencil with pictures pasted into the holes: where the control image is smooth, the sources dissolve into each other smoothly.
Any input left unconnected falls back to showing the control picture itself, so a half-wired mashup still renders something you can look at and reason about rather than dropping to black.