JPEG files
JPEG is one of the basic file types File Juicer can extract from other files or recover from flash cards.
When File Juicer extracts JPEG images they are not decompressed and recompressed, so the original quality is preserved.
JPEG images are common inside many other file formats: PDF, Microsoft Office files and browser caches, but also in raw files generated by digital cameras.
JPEG files can also contain embedded text in the form of IPTC metadata tags, added by photo applications to describe the image. File Juicer extracts these tags raw and unformatted. You can also view them in Apple Photos via the Get Info menu.
Repair of JPEG files
JPEG files contain a header that specifies how they are compressed and the dimensions of the image. If this header becomes corrupted the file becomes unreadable. To recover such a file you need to replace the header with one from a known-good JPEG taken by the same camera model at the same quality setting. Copy the header using a hex editor such as HexFiend and the file may become readable again.
Damaged JPEG files are often split into strips, so manual recovery will likely result in an image similar to the one shown here. While much of the image may be lost, some pixels can still be recovered. If you have many damaged JPEG files the process is time-consuming and usually does not justify the effort.
Preview on macOS can open lightly damaged JPEG files.
Thumbnails inside JPEG files
Most JPEG files contain a small thumbnail image in their header. If the main image data is damaged you may still be able to extract the thumbnail. It is low resolution but better than nothing. If the image came from a memory card, try flash card recovery -- the original data may still be recoverable.
Specialized data recovery companies
When disk problems are beyond what standard tools can handle, data recovery companies treat each case individually and have clean rooms for disassembling hard disks. One such company is DriveSavers.
Stop using a damaged disk as soon as possible -- every file written to it risks overwriting the data you are trying to recover. Even background processes like browser caching write hundreds of files while you work.
Badly damaged JPEG files
If you have recovered files from a memory card, try a second recovery tool as well. Data can remain recoverable long after you believe the card has been erased.
Many apparently damaged JPEG files recovered from a hard disk are not JPEG files at all but random data the recovery tool mistakenly flagged. This is hard to detect without a hex editor.
Hard disks can become corrupted in many ways, but sudden power loss is a common cause. Modern file systems like APFS and NTFS are more robust to power failures than older ones, but no file system protects against fire or theft -- take backups.
Completely empty JPEG files
Some recovery software creates a JPEG file because the file system recorded that a JPEG was once stored there, even when no actual image data remains. These files contain only zeroes. To check, use Compress in Finder -- if the file shrinks to just a few kilobytes it is empty. Real JPEG photos are typically several megabytes, though thumbnails can be as small as 20 kB.
Error messages
Firefox: The image cannot be displayed because it contains errors
If the image address is online this is likely a broken link or misconfigured server. If the file is on disk it is corrupted.
Photos: Unable to read metadata. The file may be corrupt.
Apple Photos reads metadata as its first step when importing. Without metadata it cannot import the file. Missing metadata usually means the file is badly damaged, but there may still be recoverable pixel data inside.
The "filename.jpeg" document is damaged, or isn't compatible with Pixelmator
Badly damaged JPEG images cannot be opened in Pixelmator. There may be recoverable pixels inside but the chances are not good.
Photoshop: Could not complete your request because an invalid SOS, DHT, or EOI JPEG marker is found before a JPEG SOI marker
The first header section of the JPEG file has been damaged. Pixels or a thumbnail preview might still be recoverable.
Backup
Time Machine is easy and automatic. Keep a backup disk connected and macOS handles the rest.