For an image to show up on Codepen.io it must be hosted on the web.
If you are Pro member you can save images as ‘assets’ and re-use them as you want.
If no, there are many free image hosting sites that will provide you with hyperlinks…I use Photobucket.com but, as I said, there are many others.
Finally there are ‘placeholder’image services like LoremPixel.com (http://lorempixel.com/) that will provide you with images of pretty much any size you want.
Like cats? …another is placekitten.com (http://placekitten.com/)