I used Google Forms for a survey and wanted to save the generated graphs. They have a "copy" button that copies the graph to the clipboard. I was not sure what is really in the clipboard and how to save it. After some search and experimenting I found this command to be working: 

xclip -selection clipboard -t text/html -o > out.html

It saved the content of the clipboard the out.html. The format looked like this: 

  <meta http-equiv="content-type" content="text/html; charset=utf-8">
  <img src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAC...

So there is an png image in base64 format.

If we open the HTML file in a browser we can see the image.