Joomla Templates

Is there a way for the background image to scroll down with the page until you get to the bottom of the image?

Yep there is a way. It'll require some css editing though. Right at the top of the template.css there are some rules defined for the body tag. This is where the image is attached to the body tag.

body {
margin: 0px;
padding: 0px;
border: 0px;
height: 100%;
width: 100%;
text-align: center; /* required for centering the page in IE */
background-repeat: no-repeat;
font-family: Arial, sans-serif;
font-size: 12px;
color: #E9E9E9;
line-height: 19px;
background-attachment: fixed;
background-position: left bottom;
background-color: #000000;
}

you'll need to change the bolded area to
background-attachment: scroll;
and you'll probably need a really long image. Like this site renderco.co.nz/

Very nice, thank you. Now...is there any way to define how far it scrolls, so that it stops scrolling at the bottom of my image?

No, not that I'm currently aware of sorry. There may be some fancy new css3 that could handle this....
It's a lot easier and reliable to have the back ground image fixed. Because content will vary from all sorts of heights.

You could definitely solve the issue with JavaScript but I doubt it'd be worth the effort.