Second question first. Check the wiki here in the top nav bar and the override forum and the first/only sticky on overrides. For each dir that the classic dir exists in you can create a your_template dir that will receive inly the edited files that exist in this area. move to another dir and see the classis, then those files there can edited and placed in your own dir.
For your other item, upload images to includes/templates/your_template/images
and try this code:
Code:
define('BOX_HEADING_CATEGORIES', 'url(..images/categories.gif">');