Large backgrounds wordpress

Hi, im new to the world of wordpress theme creation but not regular site creation. I understand wordpress code a little but not a lot. so please bear with me.

I have downloaded a theme and it uses a super tiny sliced full color background.png image which from what i can see in the css it repeats to form the background of the site

wp-content/themes/mythemename/images/bg.png

here is the css

font-family: Verdana, sans-serif;
	font-size: 12px;
	color: #222;
	background: url(images/bg.png) repeat-x top #2f3f4e;
	line-height: 1.45em;

Now im wanting to use a background that I have found on shutterstock which is a full grunge background with items on it

http://www.shutterstock.com/pic-20904082/stock-photo-old-compass-and-rope-on-grunge-background.html

but my concern is

1. Site load time will go through the roof
2. How to do it
3. When viewing the site at a lower resolution the fixed background image with its items will be cut off

Now i know there is a way to do this, as Ive seen people do it, where they use large backgrounds for example

http://www.press75.com/demos/urban-elements/wp-content/themes/urbanelements/images/bg.jpg

So my questions are

1. Should i just replace the small tiny bg.png with my massive sized background image?
2. How can i have this shutterstock images background appearance without having it repeat and look terrible