I once created this plugin for someone ... they needed to set backgrounds on a per-post basis.
Installation is simple:
Now all you need to do is write a post. Then add custom fields to display a background image: BGIMAGE : provide the link to your image as value. BGIMAGECSS : as value, enter the how you want the image to be displayed. Examples:
Tip: you can upload an image via the image upload function. Upon upload, you'll immediately see the link to the image you just uploaded. Copy it and paste into the custom field value.