[samuel-williams-shopify]

Slides can now declare a background image in YAML front matter. The image fills the available display surface, including space outside the 16:9 content area, while content keeps its existing layout.

---
background: images/mountains.jpg
background-size: cover
---

background-size accepts cover (the default), contain, and auto. Images are centred and do not repeat. Relative URLs resolve from the slide's directory using the same resolution as Markdown images. An explicit image overrides the template background image or gradient.

The shared renderer supports display, presenter previews, recorder, playback, and PDF export. Transitions and export readiness wait for background decoding, and failed images do not block readiness. Includes documentation, regression tests, and regenerated browser packages.

Validation:

• 332 Ruby tests and 33 JavaScript tests passed.
• RuboCop passed for changed Ruby files and guides.
• web:packages:check passed.
• Headless Chrome checks passed for wide and tall displays, presenter and recorder previews, playback, all three sizing modes, template fallback, missing images, and PDF export.