Totally by accident I came across a web page with instructions on how to prefill the "Recipient's Address" in the Snapshot postcard dialog.
The page is titled "SnapShot Upgrades Tutorial by Talia Tokugawa", at http://www.koinup.com/TaliaTokugawa/work/135784/as-list/. I put email@example.com in the recipient field, so my snapshot will automatically go to the "Snapzilla" Second Life screenshot site run by Cristiano Midnight.
You can put multiple addresses there by separating the addresses with semicolons. You could put multiple addresses and delete the ones you don't want.
I wonder if it is possible to put a dropdown list, but that's just an untested thought of mine.
Some email addresses are hard to remember or type accurately, which makes this is a nice find, in my view. Thanks to Talia Tokugawa for posting it.
Snapzilla, by the way, is part of Cristiano Midnight's SLUniverse.com site, which has a popular SL forum and other features in addition to Snapzilla.