The problem with your image scheme is that, unless you have an arsenal of images to pick from, a bot creator could easily tell the bot to find the name of the image (IE: image1.png) and submit a specific answer for each picture. This would be effective unless you had 50+ images.
Talk to iago about the text ones, he wrote a bot to find it out (I believe? Or was this someone else?) and he would know something about this (or someone who I am mistaking with iago). That's pretty vague, but I'm pretty sure that it's iago who knows about this, which should be what you need to prevent automated signups. Also, block dodgeit emails and prevent bugmenot registered emails. This is assuming you get popular enough to even have to worry about this stuff.