The moment it fails
The photos were copying from the phone or the camera card, the transfer stopped, and now the gallery shows a grey square or an error. If that picture exists nowhere else, the error looks final. Often it is not: the part of the image that arrived is usually still in the file.
Why the viewer refuses it
A JPEG starts with a header that describes the picture, then the compressed image, written from the top of the picture to the bottom, then an end marker. When a copy stops early, the end marker never arrives, and many viewers refuse the whole file instead of showing what is there. A PNG keeps its picture in compressed chunks, each with a checksum, and one damaged chunk is enough for a strict viewer to reject everything after it.
What happens to the file
The tool reads the header, follows the image data as far as it stays readable and writes a new file that ends where a file should. For a PNG it checks each chunk and picks the compressed stream up again past a damaged stretch. Nothing is painted in: the pixels that arrived are kept as they are, and the report says where the readable data stopped.
What you come away with
A photo cut short opens with its top part intact; the rows that never arrived are missing, usually shown as a grey or blank band at the bottom. A damaged stretch in the middle can leave a band of shifted or missing rows. If the start of a JPEG was lost, its header went with it and there is nothing to build from: the tool says so and writes nothing.
Where it works, from our tests
JPEG came back in every case we tested when the file was cut short or had a damaged zone. PNG came back when cut short, with a damaged zone, with shifted data and with scattered errors. A JPEG with shifted data, and either format with its start lost, did not come back in our tests. The file types guide has the full table.
Before you start
Work on a copy and keep the original. If the photo came from a camera card, stop using the card until everything on it has been copied off, because new photos can overwrite what is left. If another copy exists anywhere, in a cloud backup or a message you sent, that copy will be better than any rebuild.
Check the new photo
Open it at full size and look at the bottom and at any band where the picture changes. A photo that opens at full size is a usable file; a thumbnail that appears is not proof of that.
Common questions
Can the missing part of the photo be recovered?
No. What never reached the file cannot be rebuilt. The new photo holds what arrived.
Is the photo recompressed or changed?
No. The image data that is intact is kept as it is; only what makes the file readable is written anew.
Is the photo uploaded anywhere?
No. It is read in your browser, on your device, and never leaves it.
Sources
Documents consulted for this guide.