One way is simply to get a picture onto your clipboard (e.g. Ctrl-C while you have it selected in another program) and paste it here. I'm guessing there's a limit to how large the picture can be if you do this.
Another way is first to post the picture somewhere else on the internet—Google Photos, Facebook... I would say Flickr in the past but it's having its own issues. Then you get a URL for the hosted version of the photo, make sure that its rights are set so that anyone can see it, and then come back here and click this button:
... then paste the URL.
By the way, I just pasted that image using technique #1!
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