This has happened to me on two different WikiTree IDs. I have included one of them below.
The file is not empty, it is smaller than 10 MB and it's a jpg. I also tried it with different files, but get the same message. I logged out and back in to see if that was the issue, but that did not help.
This is not the first time that this has happened. I just didn't have the time to post the other times.
6/5/2018 7:53pm eastern
So, I left the window open and today the exact same image files loaded to Tambern-4 with absolutely no problems at all.