Be careful about pushing too hard in this direction. Non-paying users aren't getting a totally free ride (more below), and you don't want to give them a bad feeling about using your service.
Since the cost of hosting a non-paying user is actually pretty small (particularly because these aren't videos -- they're screenshots), if you have a non-paying user who embeds their useful walkthrough in a very visible location... well, that's pretty damned valuable free advertising. They've more than "paid" for the service, and you really want them to have a good feeling from the transaction, not shame them -- if they feel good about using the walkthroughs, they'll use them more, and not only evangelize but also find themselves needing the paid features... and be happy to pay for them, not forced against their will.
Actual freeloaders (people who are only using the free service in low-traffic places, or only internally) cost you almost nothing to host, so it's not smart to obsess about them.
The trick instead is to draw the right line between free and premium, and also work out how to structure pricing to account for many different kinds of uses.