Crossing from Free to Paid options

I've made a new kind of promotional page on Domai: a full thumbnail page in the free section, but just a few of the thumbnails (clearly highlighted) links to big images.

This kind of page was an attempt to replicate something I heard in a podcast.
Marco Arment, the maker of the excellent software Instapaper, talked about how people, surely partly out of sheer inertia, will keep using the free version of a thing forever, unless missing something becomes too uncomfortable. In his case, he limits the free version of Instapaper to save ten articles*, which helps a lot of people over the mental hurdle to sign up for the Pro version for five bucks.

It's an issue for a very big number of internet businesses: how to get more people to cross over from the free content to paid content.

So I'm experimenting on how to get a larger percentage of the huge number of free-bee visitors to feel that they are missing out until they actually get their membership. Showing thumbnails of the pictures they aren't getting for free is one idea, I don't yet have data on whether it works.

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*Marco says he added the 10-article limit last summer (2009). Clearly he changed his mind, in this 2008 blog post, he strongly denied a rumor that this limit existed then:

"There’s no limit of how many articles you can store with Instapaper Free [...]. There has never been such a limit. As I have previously discussed in depth, I never want to impose artificial, frustrating restrictions on Free that make it a bad product."

So I'm guessing he changed his mind at some point about whether such a limit would make the free version a Bad Product.  I'm not saying this is bad, it's a reasonable limit, it's his product, and he's free to change his mind however he wants.

I'm just wondering whether it's ten articles total, ten per day, or ten unread at any one time. I'm guessing the last one. 
(BTW, I don't have experience with the free version, because when I heard of Instapaper, I got so enthusiastic that I bought the Pro version right away (and haven't regretted it one second).)
Update: Marco informs me:

It's 10 unread articles stored on the iPhone at once. As people read and Archive them, it downloads more until it has 10.

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