I thought perhaps you could try a plugin to mimic the classic-theme functionality you’re looking for in a block theme, but I couldn’t find a plugin that would let you add a “random image” block.

You might try experimenting with this plugin:

https://www.ads-software.com/plugins/random-banner/

You could add the required shortcode via a Shortcode block. The plugin looks fairly old, but it’s been updated recently, so hopefully it still works.