File names I'm toying with to reflect the more separated nature of the bg images:

page_background.jpg (I think the convention for this is slightly different, and of course I would follow that, but off the top of my head)

entry_background.jpg
entry_header.jpg
entry_footer.jpg
entry_userpic.jpg

comment_ variation of the above (they will match as a default)

sidebar_background.jpg
sidebar_header.jpg
sidebar_highlight.jpg (this image is the "hover" that shows which page you're on in the nav module)

calendar_background.jpg
calendar_highlight.jpg (the table-head cells in the calendar module)

form.jpg (input, textarea)

A lot of these will be duplicates in a regular layout, but I think they afford a lot of customization to the end user?