Main Container Background Image in Indotherm Theme

Hi! I'm stuck on something I would like to do and was hoping I could get some help here. :)

What I want to do:
Have a background image for the main container (I think that is what it's called), i.e. the white box immediately surrounding the comic page.

What I'm working from:
I'm using the Indotherm theme, editing the tone.grawlix.css file. I thiiiiiiink I have identified the bit of code where I would have to work some magic:

/*! Main container */
.wrap {
color: #222;
background-color: #fff;
}

My question(s):
1. How do I do it? My knowledge of code is newbie level.
2. Are there any "rules" I need to be aware of when I create the background image? My main concern is getting the correct size (width and length) when designing it/not screwing up any automatic scaling that might be in place.
3. For this to work aesthetically, I'll have to remove the site banner, which I have not been able to locate in the tone.grawlix.css file. Where might I find it?

I'm looking forward to any support you can offer!

Kind Regards,

Dee