It is not necessary to get that granular about timing. Just don't check back for the pending images more than 1min, then 5min, 30min, hour, etc.

The reasons are:

1. We have a lot of users all over the world, so there is no real low volume periods

2. Unless you are talking about 10,000 new requests per hour or more; it won't really make a difference. In that case, you would have generators to handle the load, so it still wouldn't make a difference to other users.