Use proper images for thumbnails

[No767]

Summary

No longer using some svg artwork as a replacement, but actual images that are uploaded by the user. Also went ahead and updated the usages across the site to use actual thumbnails provided by users, but not done in routes/index.tsx as the events bar will get an update on itself and will be changed there.

Types of changes

What types of changes does your code introduce to the UC Merced's ACM Chapter Website?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (Updates to README.md, the documentation, etc)
• Other (if none of the other choices apply)

Checklist

Put an x in the boxes that apply

• If code changes were made then they have been tested.
• All workflows pass with my new changes
• This PR does not address a duplicate issue or PR

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[sonarqubecloud]

Quality Gate failed

Failed conditions
3.4% Duplication on New Code (required ≤ 3%)

See analysis details on SonarQube Cloud