Finally, my time to shine.
I'm gonna talk about Kengo Hanazawa (author of I am a Hero) backgrounds here.
So, how does he do those amazing bg's?, well, he uses photographs, he takes photos of backgrounds, or even photos of himself to use them for his manga, and then, he "traces" or converts them into black and white using Comic Sudio to appear more manga-like.
This is no new thing, master Asano Inio did that in Oyasumi Punpun, all the manga is drawn using real photographs as backgrounds, and that thing is beautiful to say the least.
Master Boichi also is doing this in his later chapters of Sun Ken Rock, and I could name a few other top artists that do this, or, simmilar things.
Kengo Hanazawa has talked about his method, I'm gonna leave you a link so you can check how he does his magic, it'd be better if you could speak japanese or spanish to fully understand it, but I guess just with the visuals you can graps the idea. I could give you a translation also, but that'd cost money (joking joking).
He basically talks about what I wrote previously, of him using photos, and that if you would try to draw everything "by hand" the analog way, you would take a least a week to draw one of his splash pages. But using the technology you could have this quality of manga in a weekly basis, meaning being able to keep up with a weekly chapter schedule with this level of drawing. Oh and yes, the backgrounds are done by his assistants, he mentions that in the video =P .
Enjoy!