This can happen that the product images don’t exist anymore.
The Algolia extension is relying on Magento’s cache to use the resized images, as they only exist in the pub/media/catalog/product/cache directory.
This directory is emptied each time you clean the image cache of Magento. It means that if you do so, the indexed images won’t exist anymore and you have to reindex your products to update the Algolia records with the new urls. If all your images suddenly disappear on your front end, that’s probably what happened.