Basically, this backgrounds section isn't going to ammount to much as I can only explain so much here. There's as many ways to do backgrounds as there are ways to do the overall comic. The general rule will be to choose a style that feels natural with your linework. If you are doing the background traditionally, you can use markers again or possibly watercolors. If you're going digital, you have a lot more options again.

Inked style
This is the first method. In this, you would have already sketched the backgrounds with your linework. In this case, you will have filled the backgrounds in with the coloring phase. From here on out you can either paint some colors on them in the touch-ups or just add shading as you go.

Painting
Basically, this style is painting your backgrounds like you would if you digitally painted your whole comic. Most of the time you will wait to actually paint until you're on the computer itself. The best examples I can think of this style are...well my comic. Of course if you want to see a master of it, you should check out The Phoenix Requiem.

Photoreferencing
Something very common for the painting style is working off some sort of photo reference to have a realistic feel if that's what you're going for. It can be helpful or a henderance, depending on how you work with it. You don't want to stick too much to the original photo, or rely heavily on the original details. It won't look like your style, or like a comic book at all for that matter. A perfect example of this is my 3rd page.

I had worked so hard to capture so much of the original picture that I ended up letting it just look like that...a "picture". You could barely tell I had re-painted the whole thing. I might as well have just slapped a texture on the original.

Re-using backgrounds
It may seem like cheating to re-use something you've already painted, but if it works, there's no reason to paint something new. Now I don't mean paint a scene, then at the next setting, just use the same images. I mean, if you have your characters in one spot, like mine were on the same side street for most of the first chapter, then you can re-use those moments. The characters will ocassionally be at the same angle as they were before at some point.

At other times, you can adjust the image to make a new angle as well. If you see a building in one scene then from an angle in the next, you can use the base colors of the building to paint it from teh side. You already have the foundation, and it can now promote continuity in your scenes.

Just remember never to overdo it. If you're reusing the image, you need to change it a bit as you go. Your scenes need to be from different angles anyways, so you should constantly have different backgrounds to some pont.

Thinking about your overall scene
Basically, if you're going to be creating individual frames, you want to have thought about where everyone's standing ahead of time. It's very odd to have a character on one side of the street, then suddenly on the other a couple scenes later.