when you have an image approved, it probably gets into a queue for later processing, to be added to the search database. they will wait until the server has the lowest amount of usage (off peak times) before they process the new additions to the database. this could be 24 hours later, there is no reason why it would be more if they process daily.
however, there will be search caching, so even if you are added to the database, you then have to wait for the search caches to clear, before you show up in the cached content.