I have the same problem, but i read somewhere on the internet, there is human spammers behind another spam site, they not surfing your site, but the get the images indirectly, Human users that can post the code and return back to your site, they earn money for 1000 code for example.
Image of SI Capatcha can be accessed out side of the main site, automated spam engine have users around the world and the use one site to spam many wordpress sites.
My idea make this images can not be accessed from out side the maybe check the referral code, but that engine can emulate it also.
And check the IP of who request the image with and who request the page.
