Extract thumbnails from corrupted JPGs on Mac

vlbcfcl wrote: … photos on my Mac seem to have lost the original image data.

Are these in the Photos Library for the Photos app? Then you won't be able to export the thumbnail and preview versions. If the pictures are in Photos, then you will have no idea what the file name of the picture could be. Because there could be dozens of pictures named IMG_0123, for instance, the file names in Photos are changed to GUID, 'Globally Unique Identifier' names, like F35F02F6-484D-46E3-C1DF-D356CDFF3B38. Assigned randomly with 128 bits, it's nearly impossible that two pictures will have the same name!

You can get the file name by dragging a picture into the free ExifTool Reader app to see this:

Photos says this picture's original name was IMG_2848.JPG, but that's nowhere in here.

So, your best bet may be to open the Library Package and hunt around using Quick Look to see if pictures are what you want. Do This With a Copy of the Library--Great damage can be done messing around in a Photos Library! To open the copy o your Library package, right-click and choose Show Package Contents.

Here's what you can expect

(I'm doing this with a copy. Even intending to just look might allow a disastrous mistake!) Sorting by Size will but the folders holding pictures near the top. Originals obviously has the original files-- it's worth a look, even though you think they don't exist. Resources will have thumbnail and preview versions.

A good method might be to make a new Photos Library and drag "Originals" or "Resources" into it so you can see the pictures and make decisions about what to do. You may have several versions of some pictures with different resolutions, and the Info window will let you know.

I've been playing with this a bit, and it seems to work well. Since I have the trusted 3rd party app PowerPhotos ($40,) I would use its tools to sort by size, for instance, to help get the higher resolution forms.

What do you think?

Some images might have several thumbnails with different resolutions. Extract all of them with exiftool (it is also possible to automatically extract only the best resolution):

exiftool -a -b '-Preview:*' -W '%f_%t%+.nc.%s' .

Oh sorry I misread your question. You want to grab thumbnails from Photos library package, right?

You might try to identify a certain interesting photo by dragging it to Terminal window (you need a newish macOS -- I used Monterey in this test -- AFAIR this does not work in High Sierra). In Terminal you then get text like:

/Users/<user>/Desktop/test.photoslibrary/originals/8/8B0DBF2B-6170-4316-9712-30222C4033A7.jpeg

...that is the original imported .jpg -- in Monterey Photos library the corresponding thumbnail is at:

/Users/matti/Desktop/test.photoslibrary/resources/derivatives/masters/8/8B0DBF2B-6170-4316-9712-30222C4033A7_4_5005_c.jpeg

...but that might fail if the original is nowhere to be found. Then you might try to grab all such thumbnails and see if they work.

But work on a copy or be careful not to further corrupt the library.

Matti Haveri wrote:...but that might fail if the original is nowhere to be found.

Well that's interesting! What you did with Terminal is pretty much the same as what I was doing with ExifTool Reader, of course.

I have a Referenced Library that I was playing with, and for one of the pictures dragged from Photos thumbnail view, I got this:

with the Camera file name! Others give the GUID name. This picture is editable. It's path is to the location of the Original.

A picture for which I removed the referenced original gives this:

Do I'm getting the name and path for the preview version

In a regular Library, dragging always gets me a preview version.