The best image editing application is Photoshop, but it's expensive. Among free alternatives, I'm told that Gimp is good (I haven't tried it myself).
I don't know if WP changed the way their editing tool works because I never use it (except when testing questions like yours). If it used to allow scaling upwards and now it doesn't, that's a wise decision in my opinion, for the reason I explained.
For the same reason, I'm not going to scale this particular image for you like I did with the other one: there's no way I'll become an accomplice to this!
You're unable to find a larger image of that auditorium because it's not a popular subject, and because website managers who know their job usually publish small images. Your problem is your whole idea with featured header images (coupled with your trying to use photos you find on the net instead of photos you shoot yourself). Perhaps you could abandon that idea, at least when you cannot find an appropriate image, and insert the image in the content of the page.
Or, if you use an image editing application, you could create composite images that would result in a 1000px wide final result. Simple example: