Well I'm not surprised, that an Administrator can do this. Usually they have elevated rights - and for what ever reason this seems to be a rights problem...

OK, here is what I do:

1. I go to my gallery and select a picture and click on "Edit Image"

2. I uncheck "keep current thumb" and browse for the new thumbnail

3. Then I scroll all the way down, set the checker at "I certify that this imager is...." and hit "save changes"

and then: Dang I get that error and a blank page to upload something new...

So that's what 's happening!