When File Juicer extracts JPEG, they are not decompressed and recompressed which will lose quality, but they are saved in the original quality.
JPEG's are common in many other file formats as PDF, Microsoft Office files, and browser caches, but also in "raw" files generated from professional digital cameras.
The other most common format which can be found inside a JPEG is text. This is meta tags, generated by other applications for describing the image. File Juicer extracts these tags raw and unformatted. You can also see these tags in iPhoto with the Get Info menu.
JPEG files may have PICT previews attached. Photoshop, GraphicConverter, and File Juicer can put PICT previews on images. These previews will normally "fall off" when you copy the images to Windows.
Repair of JPEG filesJPEG files have a header telling how it is compressed, and the dimensions of the image. If this gets corrupted, the file makes as little sense as an encrypted file. To read such a file again, you need a new header to unlock it. You can get such a "key" if you can get hold of an image from the same type of camera, taken with the same quality setting. Then copy the header from the good JPEG to the damaged one with a Hex-Editor (like HexEdit), and it could become good enough to be readable with image viewers.
Damaged JPEG files are often cut into strips, so manual recovery will likely result in an image like the one shown here. Most of the image is gone but some of the pixels can be recovered. If you have many damaged JPEG files this is a lot of work with results that normally can't justify the effort.
Preview which comes with Mac OS X can read lightly damaged JPEG files.
Thumbnails inside JPEG files
Most JPEG files contain thumbnail images inside. If the JPG is damaged, you may be lucky and be able to extract the low resolution thumbnail image. This is of course not the same quality as the full original, but still better than nothing. If you still have the flash card where the images was originally (if it is a photo), you may try to do flash card recovery and see if it is still there.
Specialized Data Recovery CompaniesWhen disk problems become worse than what standard tools can handle and the data you are losing is important, you can contact data recovery companies. They treat each case individually and have clean rooms if the hard disk needs to be disassembled.
2 such companies are DriveSavers and Mac Data Recovery.
They recommend that you stop using the damaged disk as soon as possible, as every file saved on it has a chance overwrite the files you have lost. Even Safari saves hundreds files temporarily while you are browsing the net.
Badly Damaged JPEG files
If you have recovered damaged files form flash cards, I recommend that you try another flash card recovery program as well. Images may be recoverable on flash cards long after you believe it has been erased.
Many JPEG files which became damaged while kept on a hard disk are not JPEG files at all but random data from the hard disk. This is not easy to see without a Hex Editor.
Hard disk can get corrupted in many ways, but loss of power or a Windows crash on FAT32 disks are prime suspects. Newer file systems like the journaled HFS+ for Mac OS X or NTFS are more robust to power failures and crashes. No matter how safe the file system is, fire and theft are other good reasons to take backup.
Completely Empty JPEG filesSome recovery software "recover" JPG data from disks where there is no data, but saves out a "JPG" file because the file system remembers that there was a JPG file there in the past. Such files contain only zeroes and nothing can be recovered from them. You can see if you have gotten a completely empty JPG file by using the "Compress" finction in Finder on it. It it shrinks to a few thousand bytes you know it is empty. JPG photos are usually millions of bytes, but thumbnails can be fine at 20.000 bytes.
Apples "Time Machine" backup is easy and uatomatic, but so is burning CDs iPhoto if you don't (yet) have a backup disk.