No doubt the images are custom - but the point is that if the network is robust enough for millions of customers to download 10.7, why is it too weak for a couple of hundred Apple stores to download images?
Oh, I'm sure the network is fine.
It's probably easier cloning a bunch of drives and shipping them from a marketing staging area along with other materials (e.g., signage, training materials, Lion t-shirts for staff) than having the local Geniuses at each retail location provision a drive from inventory and copying the downloaded image. There's probably a clean copy of the retail version of Lion on the drive, too.
After all, the Geniuses would need an external drive with various images anyhow to service their in-house systems as well as those of customers.
Apple probably has the store image available online anyhow, just in case the shipped hard drive fails, etc.
It's also possible that the shipped drives are OEM parts in inexpensive enclosures purchased in bulk, rather than a retail box version. After all, such a drive would not be really used in a customer facing situation, you can use a cheapo drive enclosure that gets the job done without looking all that purty.
Just grab a few cases of drives, a pallet of drive enclosures, a cloning station and tell the intern to get to work.
