Wait, really? Not like I'm a heavy Twitter user, but I did have an account there I used daily a few years back, and another I made just a few days ago for a bit - thus, a recent visit to the login page. And I have never seen that glitchcore stuff before.
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?
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.