Most bots have figured out how to do captchas, so now all they are is making it harder for humans to get through and easier for bots to get through. The scrambled image one could potentially work but there would have to be millions of different possibilities, and they'd have to be actual photos, so it would be very hard for a bot to analyze the pixels and figure out what the object is.
Some spammers are just too desperate to do what they do, which I don't understand because it serves absolutely no purpose. Forum spammers tend to just post some giberish and then leave. What's the point of that?