Unless your website is under targeted attack just putting "2+3" on a image will block 99.9% of all bots. You hardly even have to distort the image or randomize the math but doing so could help against script kiddies. Only drawback vs reCAPTCHA is you have to show the captcha all the time instead of automatically suspecting bots.

If you are under targeted attack by someone more dedicated, captcha is not going to be the only defense in your book.