go get github.com/go-macaron/captcha

// main.go
import (
    "github.com/go-macaron/cache"
    "github.com/go-macaron/captcha"
    "gopkg.in/macaron.v1"
)
​
func main() {
    m := macaron.Classic()
    m.Use(cache.Cacher())
    m.Use(captcha.Captchaer())
​
    m.Get("/", func(ctx *macaron.Context, cpt *captcha.Captcha) string {
        if cpt.VerifyReq(ctx.Req) {
            return "valid captcha"
        }
        return "invalid captcha"
    })
​
    m.Run()
}

<!-- templates/hello.tmpl -->
{{.Captcha.CreateHtml}}

// ...
m.Use(captcha.Captchaer(captcha.Options{
    // URL prefix of getting captcha pictures. Default is "/captcha/".
    URLPrefix:            "/captcha/",
    // Hidden input element ID. Default is "captcha_id".
    FieldIdName:        "captcha_id",
    // User input value element name in request form. Default is "captcha".
    FieldCaptchaName:    "captcha",
    // Challenge number. Default is 6.
    ChallengeNums:        6,
    // Captcha image width. Default is 240.
    Width:                240,
    // Captcha image height. Default is 80.
    Height:                80,
    // Captcha expiration time in seconds. Default is 600.
    Expiration:            600,
    // Cache key prefix captcha characters. Default is "captcha_".
    CachePrefix:        "captcha_",
}))
// ...