It depends a lot on your threat model. If the people who want a copy are determined enough, they'll just retype it :-)
Which is exactly what I want! Basically, I need to create a web page which is "humans only beyond this point". One task that humans can do easily and reliably is read messy characters. Computers can't. But computers can generate messy characters. Therefore, computers can detect whether they are interacting with other computers, or humans. There are all kinds of other threat models. I know that JYA often receives redacted stuff, and he puts it on a photocopier, and he is often able to enhance the contrast and read stuff that has been blacked out. Cool! But I'm working on a different problem. Basically, I have a web site that lets you reserve domain names before you pay for them. I want to make sure that no loser out there decides to be cool and write a script which reserves every word in the dictionary, or every sequence of eight characters, or some moronic thing like that. So I will have the page display three characters, somewhat blurry, and say, "type these characters here!" If they don't match, you're not human! (Why didn't they think of this simple method in Terminator and Blade Runner?) This same moron could sit there and type domain names all day long, but that's enough punishment in itself. This use would apply to any kind of site that lets you register (or otherwise consume resources) for free, and people might have some motive for creating an auto-script. I'm also going to use the same system on a financial system I'm working on to prevent automated transactions, as part of my anti-money-laundry effort.
So instead of having to find the black stuff on the white background, or the yellow stuff on the blue background, it's having to find the green and cyan dither stuff and the aqua and turquoise dither stuff on the blue and indigo dither background and the indigo and purple background, and further down the page you've shuffled other colors in and out of the mix.
That's a good idea. If some moron decides to somehow come up with an OCR good enough to read stuff like this: http://www.sidereal.kz/~drevil/anti-ocr.png, then I'll have to move to more advanced methods.