You might as well offer both pricing models - there's nothing wrong with one-time payee users and subscription users co-existing.There's nothing wrong with that.
While the free users are using the product, don't show them premium features, show them that this dim icon can be unlocked with a premium account or whatever (see RescueTime's mouseovers). Of course you should charge more for adding their logos. This looks like something I would sell to the university crowd - trust me, there's plenty of need there from what I've seen!
For marketing, get some hotshot instructor to use your stuff. Hmm, you should pitch to Michael Hartl to use your product for RailsCasts!