Steven, I don't think you understood the original post.
Rob, I'd add another parameter to give current and archive
filenames and temp and new filenames.
You could do some magic based on the parameters extension, but that
would be too ugly.
Currently, if a new image is uploaded, views of cached pages including
the image (before rerendering) get the other image. If the new filename
is different, they will give a 404 until those pages are reparsed.
What about changing thumb.php to give a 301 to the new name on such case?