It's just down. Not sure you're going to get a detailed root-cause analysis right now.
> Extracted the image via the GET call, did Twitter's image hosting server get compromised?
What makes you leap to that conclusion? That's just their login page.
edit: Google Image Search doesn't seem to know much about it either, with only four results, all from Pinterest [1]. I'd expect a lot more hits, if it had been Twitter's login page background for as long as you seem to suggest here. Maybe I've misunderstood what you're saying?
[1] https://www.google.com/search?sa=G&hl=en&tbs=simg%3ACAQStgEJ...