Great question — answering as the UphillTech team:
If you are looking at this email in Gmail, I believe that the googleusercontent.com was added to your image. Google recently announced that they are now caching all images shown in Gmail on their own servers as a user privacy protection. This may break some email tracking. It will certainly limit the amount of information you can gather about Gmail users that open emails. You will no longer be able to see what browser they use, for example. The announcement article gives some more information about how the Google image proxy actually works. Google serves the images from their site, but has their server request it from your server the first time it is used.
Full answer by Stephen Ostermiller: https://webmasters.stackexchange.com/a/56514 (CC BY-SA 3.0).
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