[dyladan]

You could try the reverse and float visitors credit. Once they're 5$ in the hole they can't view images anymore until they pay off the balance or something similar. Of course you'd have to find a way to make it so users can't just create a new account after they hit the 5$ limit (possibly by requiring a credit card and deduplicating by that?). Digital Ocean uses a similar model where they automatically bill your credit card once a month for accrued charges.

[ne0n]

Are there any examples of that being used successfully? My guess would be that people will just create new accounts. This might be the perfect place to test that model, though. The content is already free, you don't have to worry about losing royalties that people don't pay for if the test fails. There might need to be a strong emphasis for having an account (being able to customize or save recipes without losing data from creating a new account).

[sixQuarks]

I like this idea. You could present it as $5 of free recipes. It's like a try before you buy, or how casinos send people $100 free gambling credits. Yeah, some people will abuse it, but it's worth a try.