I dropped a PNG on the Forum post and all was well. When I dropped the second one (a script pic) the tab went black except for
And I pressed OK a dozen times and it never went away.
What may have caused this is when I dropped the second PNG nothing happened so I did it a second time. Perhaps I should have been more patient.
When I refreshed the page I saw the second PNG fine (yeah!) and also another random image of mine.
I then discovered that I had accidently selected multiple files which would explain the error message and nothing happening when I clicked OK.
But maybe the first error should stop the whole drop process.
You might want to suggest this over at discourse.org. We're unlikely to put the effort into fixing their bugs. :~/