var imgSrc = '/captcha/15799c7c7d99d9988e069fa8288e9f4a.png';
var code = '15799c7c7d99d9988e069fa8288e9f4a';

document.getElementById('captcha').src = imgSrc;
document.getElementById('captcha').style.display = 'block';
document.getElementById('md5').value = code;