Photos not showing on Chromecast

[jfdid]

Hi.
I've got the card working well on my PC but when I cast it to a Chromecast, the photos don't show up(just black). A local camera feed shows up fine on the card, so I know it's casting fine, but .JPG's don't work.
Any ideas?
Thanks very much.

[jfdid]

Card config(using as a dashboard to cast as media source):
views:

• theme: Backend-selected
  title: sss
  type: panel
  badges: []
  cards:
  • type: custom:gallery-card
    entities:
    • camera.reolinkfrontdoor
    • media-source://media_source/forslideshow/
      menu_alignment: Hidden
      maximum_files: 10
      file_name_format: '%YYY_%m_%d_%H_%M_%S'
      caption_format: '%m/%d %H:%M %p'
      slideshow_timer: 5

configuration.yaml:
homeassistant:
media_dirs:
forslideshow: /forslideshow