CSS for images: it’s all in index.php, huh? Where??

I want to tweak the CSS for the images on my site. I’m currently using the default 1.5 theme. When I opened wp-content/themes/default/style.css, I saw this comment located therein:

*** REGARDING IMAGES ***
All CSS that involves the use of images, can be found in the "index.php" file.
This is to ease installation inside subdirectories of a server.

In my browser, I viewed the source for my top-level index page (disclaimer: For whatever it’s worth, my blog is under construction, and not yet publicized. I’ve included the link in my profile here solely for troubleshooting assistance.) In that source code, I see the following:

[Moderated: code removed. Please post a link instead]

When I open the index.php file in wp-content/themes/default/, I also do not find the IMAGE CSS region.

Question: Where’s the IMAGE CSS region I included above?

Another question: Why exactly is editing the img tag CSS someplace other than in wp-content/themes/default/style.css helpful? Were it not for the embedded comment instructing one to place image css elsewhere, I’d have just put it in here.

An observation: My inability to find the IMAGE CSS region (and, to a lesser degree, my minor confusion over why I shouldn’t just place my desired img css in wp-content/themes/default/style.css) leaves me feeling that the “*** REGARDING IMAGES *** All CSS that involves the use of images, can be found in the ‘index.php’ file. This is to ease installation inside subdirectories of a server.” comment is far too vague to be of any use.