Take a look at my guide and then the chip section.
Essentially any changes to the chip itself, background, size, border, shadow, etc. Can be applied to the chip directly. But anything that needs to target something inside of the chip needs to be targeted from the main chip card and then drilled down to the appropriate chip.
Edit: also if you know what the path is to your camera snapshots you can use a template chip instead and then you could just have it change the icon when its detected motion or something, like this maybe?
type: custom:mushroom-chips-card
chips:
- type: template
entity: camera.demo_camera
picture: |-
{% if states('binary_sensor.front_door_motion') == 'on' %}
some image that will show that motion is detected.
{% else %}
local/frontdoorcamera/tmp/camera.png
{% endif %}