At many places you might have seen image verification. Do you know why image verification is given at some places. Image verification is needed to beat attackers. who might try to make the website too busy or they may keep spamming like giving comment to any blog.
There are software which will automatically do the commenting job on some website or blog a lot of time as per instruction. it is like spamming on website or blog. if you have image verification then those software can't do that repetative work. then they will have to eneter the written word in to the desired space. otherwise they shall not be able to comment on the blog or website.
i have personally got 38 same and repetative comments on a post of my blog. i had not given them image verification so they could spam. now i have given image verification and they have to enter the value in the picture to the desired space to comment on my blog. so now they can't spam on my blog.
you must enter an image verification in order to be spam free.