You will know when you have completed the challenge.
You will know because you will be told that you have completed the challenge.
At no point are you expected to contact Mr. G to verify your completion. You may contact me via the Site Inquiry line if you wish, but I will likely simply reiterate the above two lines.
(Mr. G already gave you a big hint, which I was not anticipating.)
Happy blogthday, and happy hunting!
UPDATE: We now have our nine winners! They will be notified later today. Thank you to everyone for playing.
FURTHER UPDATE: I have been asked, via Twitter, to explain the solution. Once you had all thirteen clock images, you put them in order by time displayed (with the repeats being AM vs PM following the file name - vs _). There were three characters stacked vertically on each clock face. (I have no idea how people teased them out; I hope/expected some manipulation rather than squinting.) Read in order left to right it was a URL written across three lines. Once you went to that URL it told you that you had finished and instructed you to email me some info and a unique code.