All images pending approval go into a database that spits 5 out at a time randomly for moderators to see, and then approve one and then see another random 5. Could be 30 seconds, could be a week, it's all random.
I can tell you from personal experience that well tagged pictures are often the first to be picked out for approval from the pile. So if you have a picture you really want to be approved really fast, make sure you give it upwards of 5 or 6 tags at least.
