While I am in favor of keeping things dead simple, is 6 character URLs too few? Could someone guess confidential work-related screenshots based on that size?
Also is hot-linking/embedding directly to the final image allowed or discouraged?
As for hot-linking; I have no business plan and just wanted to make something useful. I should probably figure that stuff out.
I personally think a freemium kind of model will work best for a service like this. Allow locking only for logged in users, and give each person 30 locks for free monthly (1 per day on average). For additional locks, either charge a small fee out of the user account (something under a dollar), or charge a given amount monthly for flatrate locks. You should allow people to deposit money into their user account in order to pay for these things. They'll most likely consider it playmoney, and that lowers inhibitions when it comes to buying premium features.