WDSBT-285 Sets a fallback image using a placeholder image for now

[lepittenger]

Closes https://app.clickup.com/t/9011385391/WDSBT-285

Link to test

Let me know where to deploy this to and how.

Description

Set a fallback image for if posts dont have one.

Related Tickets & Documents

https://app.clickup.com/t/9011385391/WDSBT-285

Mobile & Desktop Screenshots/Recordings

AI Assistance

• 🤖 This project was developed with the help of a LLM/AI such as Cursor, Gemini, etc.

Added to documentation?

• 📜 README.md
• 📓 ClickUp
• 🙅 No documentation needed

Added tests?

• 👍 Yes
• 🙅 No, because they aren't needed
• 🙋 No, because I need help

Testing Instructions

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Reviewer's Testing Checklist

As a reviewer, please verify that the relevant testing criteria are fulfilled and confirmed before approving this Pull Request.

• Visual Regression Testing: Ensure that existing functionality is not negatively impacted by the changes.
• Cross-Browser Compatibility: Test on major browsers (Chrome, Firefox, Safari) to ensure compatibility.
• Mobile Responsiveness: Confirm that the changes are responsive and functional on various mobile devices.
• Theme Compatibility: Ensure that the changes do not adversely affect the site's theme and styling.
• Linting: Check that the code passes all linting checks (PHPCS, ESLint, SassLint). Check if PR passes code quality check.
• Accessibility Testing: Validate that the changes comply with accessibility standards. Run npm run a11y.
• Security Best Practices: Ensure that the code follows WordPress security best practices. Check if PR passes security check.
• Documentation: Ensure that any new features or changes are appropriately documented in the README.md or Confluence.
• Post-Deployment Tasks: Check if there are any tasks that need to be performed after deployment.

[optional] Additional Reviewer Notes or Considerations?

[thatmitchcanter]

Task linked: WDSBT-285 Fallback Thumbnail for posts that don't have one?