Images missing, service unavailable
The hosting company says the only error they see is "image missing" and the DB connection is fine.


That seems suspicious to me.  Assuming the 503 Service Unavailable status code is initiated in Craft, it will do that in two common occurrences for front-end requests.  One is when Craft is in the middle of being updated, and the other is when it is having problems connecting to the database.

A look through the Craft logs as well as your web server access logs should help verify this.