This image was sent from the server to your browser with a ETag header containing an ID (instead of a traditional checksum). If the request to the image contained an ID in a "if-none-match" header, it's reused as is. If not, a new random one is generated.
By putting this image's URL on several pages of different websites, it is possible to log each visited pages for a given visitor.
Here is a list of the pages you visited that use this tracking image: