That is a good question too. It sounds like you’ve found the option in the theme option page’s background tab for the archive background image. To use a slideshow on your blog page, you’ll want to remove that. With it gone, the background should revert to the default slides that you’ve set for on that tab.
Now to add an individual background on a page/post, edit the post. Look below the main content box for another box labeled backgrounds and click the add new button. You can add in an individual background image there, or if you add more than one, it’ll be a slideshow. That’s how you can customize any of those with their own background.
The spot where that doesn’t work is the actual blog page. On that page, WordPress ignores pretty much everything other than the title. That’s why we have to add its background in the theme options.
Let me know if it gives you any trouble,
Bill