This is because the Files sharing is controlled by Chatter. Enable Chatter for the guest User profile for the site.
Once the Enabled Chatter is checked, reload the login page in a private browser window to see if that resolves the issue. If the image is still not rendering, check if 'Let guest users view asset files and CMS content available to the site' is enabled. Please see the steps for the path to follow for each user interface:
In Salesforce Classic
Setup | Customize | Digital Experiences | All Sites | Workspaces | Administration | Preferences.
In Lightning Experience:
Gear icon | Setup | Feature Settings | Digital Experiences | All Sites | Workspaces | Administration | Preferences.
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