First, you must locate and prepare the screenshot. Usually, it's in your screenshot folder (%appdata% -> .minecraft -> screenshots)
Then, you should use some sort of image uploading website or service. For a website, I'd recommend
http://imgur.com/ And for a service, I'd recommend Gyazo or Lightshot (Google them if you need to)
Upload the image, and then head to this page:
http://www.minecraftsurvivalgames.com/account/signature
In the box that will load, copy and paste the image (right click
copy, right click
paste.) And format it how and where you want it.
Preview the signature, and make sure everything looks how you want it. Then, save it and voila!