We gave a mountain photo a new sky. Its thumbnail kept the old one.

Mount Kinabalu at dawn, taken on an Android phone. We replaced the pale sky with a coloured one and saved the copy with the file's original details kept, the way many photo editors do.

• Before: our photo, straight off the phone.
• After: the sky replaced. Nothing else changed.

What changed

• The sky. Everything below the ridge is untouched.
• The thumbnail didn't. The phone stored a small preview inside the file when the photo was taken. Our saved copy kept it, pale sky and all.
• What saved the file. The copy's compression matches common software, not the phone.

What the report said: Authenticity A for the original, B for the edited copy. It showed the thumbnail it found and flagged that it no longer matched the picture.

Go deeper

• How to check one yourself: Has this photo been edited?
• Why it works: The EXIF thumbnail that gave the edit away, by Kenneth Springer
• Try it on your own photo