I will also keep thinking about this. The only problem is that I don't have any experience with hacking sites - if anyone has got this experience perhaps you could help to explain how systems like this can be bypassed in order for me to know how secure the systems need to be. From the comments made so far it seems that conventional captchas can be hacked without too much trouble which makes me wonder why they are still used so widely? I suppose some protection is better than none......
Here's an idea. I remember seeing some jokes email with a gif of this Dutch girl band. When you did a 'Select All' it would get selected and go blue-ish - upon doing that you could basically see them naked. Maybe the same trick can be used? It'll have to do with the hue and all that of the colours. Once mixed with blue it basically changed. But asking a non-whizz to select a picture could again be too much. Just thinking out loud...
You could use JS to select the picture? But then I suppose you would need users to have JS and therefore you could just block anyone without JS like the bots.
Good thinking, JS can do it (but like you said not everyone allows it). Seems like whatever we think of has a catch