How to create an image on the fly with Node.js?

I'm using Node.js and Express web framework, I need to create an on-the-fly image .png or .jpg (like captcha), then I have to send that image to the browser.

What is the simplest way to do that? In the image I should write letters/numbers (max 5).

P.S. I don't know what library to use that connects to an online service as the recaptcha module does.