Your captcha is useless and annoying. There are better ways to prevent oracle attacks.
If anyone is interested in breaking this style of captcha, the basic technique is: intensity histogram, contrast, despeckle, horizontal histogram, cut letters/find hulls, unrotate and rescale, build grid, lookup in database. Building the database in advance is the hardest part, and it is really only hard if the site under protection isn't worth getting into.
Captchas were great 10 years ago, when not everyone knew how to break them. By now, they have to be nearly unreadable to be effective. Within a few years, I suspect that getting a correct answer for a difficult captcha will be taken as evidence against the humanness of the interpreter.
Well, that's one way to do it. Much easier is paying a kid in China $0.01 to do it for you.