When you get sent a webmention and it contains a h-card/u-photo, IndieBlocks will attempt to download the image and store it locally.
By default, avatars are stored in wp-content/uploads/indieblocks-avatars.
If, later on, IndieBlocks encounters an image with the same URL and the local copy is over a month old, it will re-download and overwrite the previously stored avatar. Newer images are not updated.
If a local avatar is ever deleted, it will be “replaced,” on your site’s front end, by either your WordPress’ default avatar or the “mystery man” image.