It makes you wonder if there was supposed to be some plot reveal or exposition explaining that Holmes had additional information that told him it was a fake painted in 1858.

Making the supernova large in the painting was probably just so those playing at home could see it on screen, although I agree that any time a TV show uses that kind of technique, it comes across as sloppy and unconvincing. There are so many other, better ways to convey clues.