Rated 4 out of 5 stars

totally great! Some room for improvement:
* include that by default in Thunderbird
* use OS-specific paths, too - example: ~/Library/Images/People/ can contain image files on MacOS with filenames identical to the corresponding e-mail addresses (suffix .tiff) which are then displayed in the native app. These could also be used here (?)
* prefer using contact images if they are in the address book over site-wide images