JPEG images massively corrupted, all stripes no image - any ideas?

Because Mac OS does not have the ability to modify file types unless a user intervenes and you have stated that is not happening. As mentioned, the issue is likely Adobe.

It will help us get a good idea of what is causing your problem if you provide a report of your system, that way we do not have to play 20 questions with you. The report we are requesting does NOT provide any personal information and is extremely safe to use. Please navigate to www.Etrecheck.com and download the free version of EtreCheck. Once you have you downloaded the app and installed it, please run the report and save it. This report will help us get a good idea what has been installed on your system and help us be able to diagnose what may be wrong.

When you have your report, you can attach it when you reply to this message and we can then review it and help you determine what is needed to get your system running well again. 

For instructions on how to download your EtreCheck report and attach it to your reply to this message please click How to use the Add Text Feature When Post… - Apple Community It is helpful to us if you download the report to your Desktop.

Mac OS has no way of damaging your files. Your screen shot indicates the file was created July 2nd so just use your Time Machine backup to restore an earlier version of the file(s). Your information also indicates Photos 2020 was used to create the content, so I would STRONGLY recommend that you search on a Photoshop forum or contact Adobe for support on their product. This is NOT a Mac or Mac OS issue, it is an Adobe issue.

EtreCheck was developed by a very trusted member of these communities and is safe to download and use. As mentioned it does not show any personal information and simply tells us what has been installed and displays clues to issues. Feel free to ask on these forums if EtreCheck is safe, you will find it is.

Members of these communities have been using EtreCheck for many years now, however if you don't want to download and use it I totally understand and wish you luck finding a solution to your problem.