code_img, strs = create_validate_code()
session['code_text'] = strs
buf = StringIO.StringIO()
code_img.save(buf, 'JPEG', quality=70)

buf_str = buf.getvalue()
response = app.make_response(buf_str)
response.headers['Content-Type'] = 'image/jpeg'
return response

document.getElementById("vericode")
        .setAttribute('src','/verifycode?random='+Math.random());

<div class="form-group">
    <label for="form-code">验证码
    <a href="#"><img onclick="this.setAttribute('src','/verifycode?random='+Math.random())" src="/verifycode" title="点击重新获取" /></a></label>
    <input id="code" name="code" type="text" value="">
</div>